Manchester United have had so many different centre-back pairings this season it's hard to keep track of them all.

It's safe to say that United haven't been at their best defensively this season having already conceded 23 goals in all competitions.

A lot of fans were of the belief the team were leaking goals because of the system, with Ole Gunnar Solskjaer's trusty 4-2-3-1 formation failing to cut the mustard, yet after switching to a back three against Spurs and grabbing three much-needed points, the United boss again played a 3-5-2 against Atalanta on Tuesday night.

In all honesty, I'm actually staggered it took Solskjaer's job to be on the line for him to switch formations because some of the errors his backline have been making this season have looked like the sort of things you see in a school playground.

Solskjaer had the same problem last season with only Leeds United conceding more goals from set-pieces than United, a stat that undoubtedly played a part in the club's pursuit of Raphael Varane.

However, the Frenchman has had his fair share of injury problems throughout his career, and that trend doesn't look set to be bucked anytime soon after he hobbled off in Bergamo on Tuesday night having only recently returned from a groin strain.

If Solskjaer wants to play with three at the back in the Manchester derby this weekend, it seems pretty evident he's not going to be able to call upon his favoured back three of Varane, Harry Maguire and Victor Lindelof.

Before the Atalanta clash, I'd already voiced my concerns on Solskjaer playing with a back three considering the options available to him. After all, Eric Bailly hasn't played much of late and Phil Jones hasn't played in about 30 years.

Forgotten £30m man proves Manchester United boss Ole Gunnar Solskjaer has made a huge mistake

I think we can all comfortably say that Bailly is Solskjaer's fourth-choice centre-back. Before the Atalanta game, he'd only made one appearance this season as United were dumped out of the Carabao Cup by West Ham United.

There's been a lot of speculation linking Bailly with a move away from Old Trafford, something that probably didn't come as too much of a surprise given his lack of game time.

There was a time earlier this season when I thought Bailly's career at United was over. When United travelled to Leicester last month, Maguire had only just returned to training having been sidelined through injury. With Varane unavailable, a lot of fans were calling for Bailly to be handed his opportunity, but instead, Solskjaer opted to rush Maguire back into the side - we all know how that went.

The decision looked to be the final nail in the coffin for Bailly, yet the trip to Atalanta saw the Ivory Coast international handed a start after Lindelof picked up an injury in training, and Bailly's performance on the night definitely proved Solskjaer wrong for overlooking him at the King Power Stadium.

United could easily have conceded five or six without Bailly at the back. The former Villarreal man put in a titanic display, with his last-ditch block to deny Duván Zapata in the first half almost as good as a goal in itself.

It was a performance that highlighted why he has to start more games, especially when you think about how badly some of United's other defenders have played this season.

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Can I see Solskjaer giving him more game time? I'm not so sure he still truly trusts him. That being said, the Norwegian has always shown that he's willing to give chances to players who strive to prove him wrong and Bailly definitely did that on Tuesday evening.

I'm a firm believer Bailly needs to start in the Manchester derby ahead of Maguire because United's captain has been awful this season, and Varane's injury may just have offered him a route back into the team.

What did you think of Bailly's performance? Follow our UnitedOn My Mind writer Omar Garrick on Twitter to get involved in the discussion and give us your thoughts on the Atalanta match in the comments section below.