Although it's impossible to argue that (too) many of our players were underperforming, the creative heart of the team had been ripped out of it by injuries. Players that have some very good attributes were unable to use them because they weren't given the service they need. The strikers for example have been feeding off scraps for most of the season. They have been getting one chance per game or less. Last season they could "afford to be wasteful" because they knew that another opportunity would arrive soon after.
The arrival of Rooney and the return of Huddlestone is a massive boost for the team. Add to that the emergence of Knight over the last few games after a settling in period early on, and suddenly chances are being created. Marriott on Thursday missed three or four sitters. The chances kept coming and he finally tucked one away. Waghorn also has missed many chances recently, but through sheer weight of numbers also scored. I wouldn't say that either of them are playing to their potential, but certainly look better because they are being given more service.
I would say that perhaps many of the players weren't actually underperforming, more a case of they were being under provided. Mount and Wilson scored loads of goals between them last season, but they also provided loads of opportunities for others, either directly or as part of the buildup. Having an effective midfield is imperative for an effective team. When three or four players in that area are producing the goods then other players around them are going to look better also because of the opportunities that they provide.
We have still got unfortunately too many average to slightly better than average players. They look much better with good players around them. Unfortunately they aren't good enough to carry the team without the others. A case of the sum being better than the individuals.