After being without his best player, Paul Pogba, for two months, this long-term injury to another key member of the squad is a huge blow. The manager loves to find an excuse for his failings but it would be unfair not to acknowledge that United have been unfortunate when it's come to injuries this season.

Rivals Manchester City have had their most important players, Kevin De Bruyne and David Silva, fit for every Premier League game this season, while Pogba has missed morethan half United's league matches. Mourinho's back line has been hit with injuries throughout the course of the season too, with Chris Smalling and Phil Jones, the latter in particular, again proving they're made out of glass. But the repeated injuries to Bailly have been the most damaging.

The Ivorian may have an unconventional approach to defending, often winning the ball with his body contorted in to the most unusual of positions, but he has quickly proven to be a fan favourite thanks to his no-nonsense style.

United have been desperate to replace Nemanja Vidic and Rio Ferdinand for a few years and Bailly has come the closest to looking like a defender capable of reaching their level. Still only 23, he is years away from his peak and looks set to have a great future ahead too, but his troubling fitness is a concern.

Bailly was injured on six different occasions last season, missing United's games against City in the cup, Arsenal away, Liverpool at home, among many others, as a result. This season, he has played in just 10 of United's 26 games in all competitions, and could now be facing ankle surgery that would see him ruled out for some time yet.

His last appearance was more than a month ago after returning from international duty with the injury. He has played in a couple of matches since, with Mourinho revealing the player has been receiving some conservative treatment, but with that not having the desired effect, it looks as though surgery might be the last option.

With David De Gea having such a fantastic season, the loss of Bailly isn't as damaging as it could be, but it does mean that United will have been without their best defender for most of the season.

United have conceded just four goals in the 10 games he's played. The fact that Marcos Rojo, United's second best defender of last season after Bailly, didn't play a single game until the end of November, and is still being eased back in, has only compounded United's frustrations at the back. The Argentinian went off with an injury at half-time against City and was missing from the squad for their midweek win over Bournemouth.

All teams suffer injuries and that is where squads come in to play, but it's hard to get away from the fact that United have struggled more than many, and certainly more than the only team above them in the league.

Over this busy festive period, where United will play six games in a fortnight, they really could have done without this latest Bailly news.

They will now have to hope that Jones, Smalling and Victor Lindelof can stay fit for the next few weeks, with Rojo's return date yet to be set. Yet fatigue could become a problem, with Smalling having already played 90 minutes for the past 17 consecutive games. That works out at a full game every four days, which isn't ideal preparation for this busy run-in.

Like Mourinho has begun to trust academy graduate Scott McTominay with more playing time, with him starting against Bournemouth midweek, supporters will be hoping that Axel Tuanzebe is afforded the same chance in some of the easier fixtures.

But if Rojo doesn't return soon and one of the other defenders gets injured, Mourinho may have little choice but to dip in to the reserves.

Bailly will be a big miss and now it's up to the other defenders, and De Gea, to ensure United hold on to the best defensive record in the league. It won't be easy though.