With Premier League clubs unable to make any more signings and the window to sell players to European clubs set to slam shut in under a fortnight, it's time to reflect on how the English top flight's biggest sides have fared during the summer.

It was a difficult transfer window for Manchester United, one that contained plenty of not-so-subtle digs from Jose Mourinho as the Old Trafford outfit struggled to bring bodies through the door. Fred was their single genuine first-team addition, with fellow signings Lee Grant and Diogo Dalot only really adding to the depth of the squad.

So, where did United fall short most this summer? Football FanCast's expert writers attempt to answer a simple question - what will prove to be the Red Devils' biggest mistake of the transfer window?

James Beavis - No alternative to Matic

"While Man United's centre-backs had a bit of a shocker against Brighton on Sunday, Mourinho's failure to sign a player similar to Nemanja Matic - who is arguably United's most important player - is one that could come back to haunt him with the Serbian currently absent. The Red Devils only conceded 28 goals last season which proves that Eric Bailly, Victor Lindelof, Chris Smalling and Phil Jones are not the problem at the heart of the defence as long as they have good protection in front of them. That is something that new boy Fred and Andreas Pereira don't give them, meaning Mourinho's trusted defensively-organised system is being picked apart by their opponents right now."

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Danielle Joynson - Failure to sign a centre-back

"It may be the obvious answer, but Manchester United's biggest mistake was failing to sign a centre-back. The calamitous defending during last weekend's defeat to Brighton highlighted how badly the Red Devils need a leader at the back. United certainly have the funds to land the likes of Toby Alderweireld or Harry Maguire, but for some reason they were unable to get a deal over the line. Eric Bailly is by far their best centre-back, and even though he had an off day against Brighton, he is still their most solid option. It is unlikely that United will be as bad as they were defensively against Brighton as the season progresses, but signing an established defender would have made life that little bit easier for Mourinho and co."

Greg Potts - Mourinho going public

"United’s biggest mistake during this window was actually more of an individual error by Mourinho. The club had already signed Fred, Diogo Dalot and back-up keeper Lee Grant when the Portuguese boss came out and moaned that he had given the club a list of five transfers, of which they were yet to complete any. This effectively told the three signings they ended up with that they weren’t wanted by their manager and also highlighted the unrest between Mourinho and Ed Woodward behind the scenes. After that, Mourinho’s further public calls for signings would have damaged the confidence of the players already at the club. It wouldn’t surprise me if his insistence on signing a centre-back during the window had something to do with the nervous performances of Bailly and Lindelof against Brighton."

Will Jones - Should have signed a playmaker

"For all of Mourinho’s pining for a new centre-back, there is no evidence to suggest that any new arrival in this position wouldn’t simply devolve into yet another ill-disciplined and baffled-looking defender charging around witlessly at Old Trafford. At what point does this succession of failed centre-back signings become the manager’s fault?

"That’s not to suggest that the back line is United’s only problem at present. The Red Devils can boast an impressive array off attacking talents, but without a playmaker capable of carving out goalscoring opportunities for them, those big forward names are as useless as, well, Chris Smalling.

"Getting the best out of the likes of Romelu Lukaku, Alexis Sanchez, Anthony Martial et al. requires the presence of a top class creator in the mould of Christian Eriksen. Juan Mata is no longer that man, Jesse Lingard never will be, and United have now missed the opportunity to bring in someone who is."