Manchester United had a mixed campaign last term. On the one hand, they finished the season in second place in the Premier League, which can be considered progress from their third-place finish the season before.However, their capitulating demotion from the UEFA Champions League and subsequent failure to win the Europa League represented major low points.Nevertheless, there were enough positive signs to give fans hope for the upcoming campaign. The performances of players like Bruno Fernandes, Harry Maguire and Luke Shaw also gave room for optimism.Some of the players have begun training at the Carrington ground to get back in shape for pre-season in light of the resumption of competitive football next month.The club announced five pre-season fixtures lined up across three weeks, with a 2-1 victory over Derby County having already been recorded.Several first-team players at Manchester United progressed to the latter stages of the European Championship with their nations and have, in turn, been given extended breaks.📺 #MUTV is the place to be for our second pre-season test of the summer! ⤵️#MUFC— Manchester United (@ManUtd) July 23, 2021In light of this, Ole Gunnar Solskjaer has been training with a squad composed mainly of the Under-23 side. However, a number of senior players, including Nemanja Matic, Jesse Lingard and Juan Mata, have already begun training.Pre-season is primarily made up of exhibition games to get the players back in shape for the grueling season ahead. However, it also offers some players on the fringes of the squad the chance to show their worth in the absence of more established regulars.A lot of Manchester United players have a lot to prove in pre-season and are likely to give their all to make a positive impression on Solskjaer.Here are five Manchester United players to watch in pre-season.#5 Axel TuanzebeAxel Tuanzebe has a lot to prove in defenseAxel Tuanzebe came through the Manchester United academy and made his first-team debut in 2015. Since then, he has been on the peripheries of the first team and underwent two separate loan spells with Aston Villa.Harry Maguire is Manchester United's captain and bonafide defensive leader. However, the spot opposite him in defense has been rotated between several players, including Victor Lindelof and Eric Bailly.The absence of both men gives Axel Tuanzebe a window of opportunity to make a positive impression in pre-season. Failure to do so could prompt another loan spell or direct sale, especially with the proposed impending arrival of Raphael Varane.#4 Facundo PellistriFacundo Pellistri will get a chance to impress with the senior teamFacundo Pellistri made a name for himself in his native Uruguay before joining Manchester United in the summer of 2020. A loan spell to Alaves was completed in January 2021 to allow the teenager to get attuned to the demands of senior football. View this post on Instagram A post shared by Manchester United (@manchesterunited)This pre-season offers Manchester United fans an opportunity to get an up-close glimpse of the highly-rated teenager.A positive pre-season performance could go a long way in determining whether Pellistri spends this season with the senior Manchester United squad or continues his development elsewhere.On that note, he has started positively by scoring the match-winner against Derby County.Also See: Fabrizio Romano's Transfer Roundup: Arsenal's plans for midfield, Update on Barcelona's plans for Griezmann and more