Manchester United kick start their 2021-22 Premier League campaign with a huge fixture against bitter rivals Leeds United at Old Trafford on Saturday. After a couple of months of lull, it is finally time for on-field action to take centre stage. The atmosphere inside the Theater of Dreams is expected to be electric given the slackening of the restrictions.Manchester United's summer transfer activity has been nothing less than impressive, with two high-profile signings already being made. As confirmed by manager Ole Gunnar Solskjaer, neither Jadon Sancho nor Raphael Varane is expected to make their debuts against Marcelo Bielsa's men. However, the club faithful could still start weaving their dreams surrounding the duo.It would be wrong to get the hopes too high though. The Premier League is a very physically demanding league with the competition getting higher every season. Teams such as Leicester City and West Ham have started to threaten the traditional top-six in the Premier League and any lapse in concentration could prove detrimental to United.Manchester United have struggled for consistency in the Premier LeagueManchester United have to address their issues with inconsistency. Although Solskjaer's team have finished their last two Premier League seasons on a positive note, they have always been forced to play catch-up. Last season, when they started to pick up pace, the race for the title was almost already over.Ahead of the upcoming season, here's a closer look at the five main obstacles that could prevent Manchester United from ending their Premier League title drought.#5 Absence of a high-profile centre-forwardVillarreal CF v Manchester United - UEFA Europa League FinalManchester United's scoring hopes have rested hugely on the shoulders of an aging Edinson Cavani. Although the young Mason Greenwood will get his chance, the 19-year-old is only expected to contribute in part.While Solskjaer is more keen on finding scorers from all across the pitch, it still helps to have a consistent centre-forward among your ranks. Cavani, with his advancing years, might still have a year left in his barrel. However, he might become prone to injuries and in his absence Manchester United's goal scoring hopes could suffer.Edinson Cavani has signed his contract extension with Manchester United, here we go! 🚨🇺🇾 #MUFCThe agreement has been completed, Cavani has accepted to stay after Man Utd board and Solskjaer were pushing to keep him. Confirmed and done deal. 🔴 #ManUtd #Cavani— Fabrizio Romano (@FabrizioRomano) May 10, 2021Manchester United have made a huge investment in signing two players. That means that the Red Devils do not have the adequate budget to conclude a deal for a world-class striker.And with Chelsea and Manchester City investing heavily in their strikers, Manchester United's Premier League chances could take a hit.#4 Over-reliance on Fernandes, Sancho for creative sparkManchester United v Everton - Pre-season FriendlyManchester United's new Portuguese superstar Bruno Fernandes has hardly put a foot wrong since his move to the Premier League. He has been a constant presence in the middle of the pitch, featuring in all but one Premier League game last season.With the blockbuster signing of Jadon Sancho from Borussia Dortmund, Solskjaer is looking to complement Bruno Fernandes inside the attacking third.🚨 Manchester United complete Jadon Sancho signing #mufc #mujournal https://t.co/FQv1tcIRPO— United Journal (@theutdjournal) July 23, 2021Together the duo scored 44 goals between them last season, though in different leagues. Manchester United are looking for something similar from them this season as well. But if the combination fails to deliver, they could be in for trouble because Solskjaer hardly has a back-up plan.Then again, even if either of the two suffer from a long-term injury, Manchester United's plans could go out of the window.Another of their go-to guys for goals, Marcus Rashford, is expected to miss the opening part of the season due to injury. Furthermore, Anthony Martial is badly out of form and has been linked with a move away from the club.As such, the onus will be on Fernandes and Sancho to offer a creative spark. Should they fail to do so, Manchester United could struggle over the course of the season.