It was a poor job from the coaching staff all game I thought. In the end, we just kind of let Southampton have free reign of the midfield(even before the Williams injury).
The better move might have been taking off Greenwood(who couldn't get into the game) for Fred and then you shore up the midfield with an additional player and ease up Pogba's job.
I just thought it was obvious.
We had no midfield and we also had a "midfielder" sitting in with the centrebacks doing absolutely nothing. You can make a change there with absolutely no risk of it having a negative effect on the team because you have a player in one area struggling and a player in another area not doing anything.
I was surprised Matic even started because there's no way Matic and Pogba as a pair are suited to a team who sustain a high press, which it was blatant that Southampton would do because they literally always do. The plan to deal with that was to take Matic out of midfield...I mean that part is fair enough but that means you don't play him. You don't handicap the entire team just to accomodate him elsewhere.
It's the sort of thing that leads to problems down the line as well because Fred/Mctominay can't be happy to sit on the bench every week, and then they see that even when the person playing instead of them can't do their job and it causes the team to struggle, they still sit on the bench.
I had no real problem with Greenwood as his presence and movement was giving Rashford and Martial more space, and there was no reason why we shouldn't have been able to get the ball to them.
I get that earlier in the season we didn't necessarily have the players to be effective but now everyone is fit and we have strong numbers in midfield, so there's no excuse to handicap ourselves in games.