A little over 100 days have passed since Manchester United appointed German tactician Ralf Rangnick as their interim boss last year.After a nightmarish start to the season under Ole Gunnar Solskjaer, Rangnick's appointment was accompanied by renewed optimism among the Old Trafford faithful.Patrick Timmons@PatrickTimmons15 years without a trophy for Manchester United. 1,825 days.3:24 AM · Mar 16, 202296649415 years without a trophy for Manchester United. 1,825 days.Unfortunately, United have not shown much improvement under the former RB Leipzig boss, especially at the back, where their calamitous defence continues to leak goals.Defeats to Middlesbrough and Atletico Madrid in the FA Cup and UEFA Champions League respectively, mean Manchester United are set to end trophyless for a fifth straight season. They are out of the Carabao Cup as well and trail league leaders Manchester City by 20 points with only nine games left. United trail fourth-placed Arsenal by four points, having played a game more.Manchester United's superstars have faltered under Ralf RangnickThe German manager has been at the helm at Old Trafford for 20 games so far, with United winning nine, drawing seven and losing four. With an underwhelming win percentage of 47.5, Rangnick is unlikely to be given the job permanently.Fabrizio Romano@FabrizioRomanoRalf Rangnick on permanent Man United job: “I have no idea. I am here until the end of the season and it is important to develop the players”. #MUFC“We have had one or two meetings with the head of scouting. We have not spoken about the new manager too much”.7:41 AM · Mar 4, 2022186041105Ralf Rangnick on permanent Man United job: “I have no idea. I am here until the end of the season and it is important to develop the players”. 🔴 #MUFC“We have had one or two meetings with the head of scouting. We have not spoken about the new manager too much”. https://t.co/AdH7g9VB3aMeanwhile, the Red Devils' defence continues to entertain rival supporters with a myriad of mistakes. United have conceded 20 goals in 20 games under Rangnick, although their attack (28 goals scored) hasn't fared much better either.On that note, here's a look at five Manchester United players who have struggled under the interim manager:#5 Raphael VaraneRaphael Varane celebrates the win against Tottenham Hotspur.Manchester United's signing of 28-year-old Frenchman Raphael Varane for £36 million from Real Madrid last summer was viewed as a bargain at the time. Fans expected the centre-back to stamp his authority on a backline whose struggles dated back to the 2020-21 campaign.Harry Maguire's well-documented struggles have overshadowed several strangely tentative displays from Varane. The Frenchman has featured 15 times under Rangnick, yet United have kept just three clean sheets with Varane in the lineup.He has also missed 14 games this season due to a plethora of reasons, including hamstring and groin injuries.Simon Stone@sistoney67Of all the various probs @ManUtd, Raphael Varane's repeated unavailability has been significant. Apart from one decent run from Newcastle (a) to Southampton (h) he hardly ever plays more than two games in a row. He was supposed to be solving central defensive issues.6:21 AM · Mar 7, 20224583346Of all the various probs @ManUtd, Raphael Varane's repeated unavailability has been significant. Apart from one decent run from Newcastle (a) to Southampton (h) he hardly ever plays more than two games in a row. He was supposed to be solving central defensive issues.Varane has been an unrecognisable shadow of a defender who once displayed remarkable composure, winning the FIFA World Cup and four Champions League titles with Madrid.#4 Diogo DalotDiogo Dalot drives forward with the ball for United.The struggles of £50 million man Aaron Wan-Bissaka this season have afforded a consistent run of games for 23-year-old Portuguese wing-back Diogo Dalot. However, the former AC Milan defender has been unable to seize the opportunity, and United's already weak right flank has suffered as a result.DavidMeyler@DavidMeylerThat’s Dalot’s problem right there. Never checked his shoulder & wasn’t aware of Lodi. Poor goal to concede.2:15 AM · Mar 16, 20221972That’s Dalot’s problem right there. Never checked his shoulder & wasn’t aware of Lodi. Poor goal to concede.Rangnick has persisted with the Portuguese defender, who has played 14 games without registering a single goal involvement.Dalot also lost Renan Lodi at the back post during the second leg of United's Champions League tie against Atletico Madrid. The Brazilian full-back was gifted a free header that eventually dumped the Red Devils out of the competition.United may have no choice but to bring in another option at right full-back this summer.#3 Cristiano RonaldoCristiano Ronaldo couldn't save United in the UEFA Champions League.Apart from a sensational hat-trick in the 3-2 Premier League win over Tottenham Hotspur, Cristiano Ronaldo has largely struggled in the second half of the campaign.Excluding that terrific display, the Portuguese star has scored just three goals in the other 15 games he has played under Rangnick. Of Ronaldo's 18 strikes this season, only six have come under the interim coach.The dry spell includes a crucial penalty miss against Middlesbrough in the FA Cup that would eventually see the game go to a shootout, which United lost. Ronaldo was also remarkably subdued across 180 minutes against Atletico Madrid, barely getting a sniff at the La Liga team's goal.UEFA Champions League@ChampionsLeagueNo Lionel Messi and Cristiano Ronaldo in Friday's quarter-final draw. The duo miss out on the last-8 for the second season running.#UCLdraw4:20 AM · Mar 16, 20226922661No Lionel Messi and Cristiano Ronaldo in Friday's quarter-final draw. The duo miss out on the last-8 for the second season running.#UCLdraw https://t.co/DqrqsD18Y1His theatrical reactions to being substituted, as well as a reportedly unsanctioned trip to Portugal ( as per The Athletic) during the Manchester Derby, have hardly helped matters. The club's highest-paid star and one of the greatest players to have graced the game has been simply not good enough this season.#2 Marcus RashfordMarcus Rashford is enduring the worst spell of his career so far.Marcus Rashford has endured a baffling fall from grace over the last year. The 24-year-old had 21 goals and 15 assists across competitions for Manchester United in 2020-21 and missed just one league game all season.This season, the Englishman has only five goals and two assists in 26 appearances and is reportedly considering his future (as per Fabrizio Romano) at the club. Rashford has played 16 games under Rangnick, scoring just twice and assisting once.From poor decision-making to constantly giving the ball away, the forward has been woeful whenever given a chance to impress.MailOnline Sport@MailSportMarcus Rashford denies making rude gesture to Manchester United fans following defeat to Atletico Madrid trib.al/GGWiSj43:15 AM · Mar 17, 202283Marcus Rashford denies making rude gesture to Manchester United fans following defeat to Atletico Madrid trib.al/GGWiSj4His body language and obvious frustration has been the most concerning, though. The Englishman hardly looks like a player who is enjoying his football and his situation at Manchester United.#1 Harry MaguireHarry Maguire is woefully out of form at Manchester United.It is getting to the point where one can't help but feel sorry for Harry Maguire. There is an argument to be made that Manchester United had no business spending nearly £80 million on the 29-year-old English centre-back.The most expensive defender in football history has endured a miserable campaign. There are doubts over his place in the starting XI and whether he is deserving of the club's captaincy.GOAL@goal2022 has not been kind to Harry Maguire 12:35 PM · Mar 13, 20223036825292022 has not been kind to Harry Maguire 😫 https://t.co/Daj83TMkB2Maguire's 15 games under Rangnick have been filled with comical errors - an own goal against Spurs, a gift of a goal to Manchester City, and a clash of heads with his own teammate, Cristiano Ronaldo.While it is difficult to predict what the future holds for the England international, one can't help but wonder if Manchester United would be better off letting him go.