Mourinho restricted United and in turn that fucked off the players, negative tactics and no enthusiasm lead to some terrible performances. All Solksjaer has done so far is give them freedom, they're happy now and so they're showing enthusiasm to go and do their thing, when they come up against less talented squads that will generally be enough to see them home. I don't see that being anywhere near enough against City or Liverpool where they haven't got a big excess of talent over their opponent, hell I think they'd really struggle against Chelsea or Arsenal who look a lot more settled tactically. Even today against Bournemouth they were exposed at times, they were somewhat fortunate to only concede the once, a tactically disciplined side won't be so susceptible to Pogba's happy go lucky approach and they'll certainly find ways to punish it going the other way.