I actually think the main issue we had against Southampton was fatigue. In the first half, after about 20 minutes, we got our heads right, started to make the pitch wider and go via the fullbacks into midfield. That allowed us to repeatedly beat the Southampton press and score both our goals. The real issue early on was Lindelof and Maguire dawdling on the ball and then hoping for Pogba or Matic to bail them out.
However, in the second half there was a noticeable drop off and I think that's just because we've played too much football lately. Our best players looked knackered. Fernandes looked especially leggy, Pogba wasn't far behind. Only the forwards looked like they could've kept going really, which is admirable given how much work Martial put in. However, Martial has been rested regularly thanks to Ighalo. Fernandes has pretty much played every minute, including 120 minutes against Norwich and even against Southampton he stayed on for over 70 minutes. I get Ole doing that, without Fernandes teams don't fear us and it just invites pressure. But it meant that a lot of tired passes were played in the second half and that gave Southampton belief that they could capitalise on our errors.
Basically Monday night's game just shows why actually we do need to buy Thiago or Grealish or someone. Nobody fears McTominay and Fred. For good reason, they can't keep the ball. We need better depth so we can rotate the likes of Pogba and Fernandes, who we risk running into the ground before the season is over and the silverware is handed out.