United are weird.
They played well in the first half (as usual) but Southampton still had good chances, then Southampton came on strong in the second.
I think the pundits are right, United players come alive when they have the ball in good positions but are completely non committal when they are out of possession. Southampton ran 5k more than them.
Overall us and United are in a transitional phase (United forever it seems) and you are always vunerable to a team that are not, and very much playing to 100% of their managers plan. Lesser players, but playing to greater than the sum of their parts. It takes a lot of effort to play the way Southampton do, so perhaps they're near the top of a peak fitness cycle at the moment, who knows?
It's galling though as it's a great thing to watch when your team plays like that.