They aren't at all decent going forward, that is the problem. Rooney is pretty much their one-man team and van Persie occasionally does something.
Vidic isn't half the player he used to be, but he's still doing something of a decent job, at least for a top half side, and so is Evans. De Gea is their best young player and worthy of playing for a title-challenging side when you consider his potential. Evra is the main problem in defence but their attack and midfield are so much worse.
The problem is that buying any players won't do much. They first need to hold onto van Persie and Rooney, especially the latter, and I can't see either sticking around past the summer, with them leaving in January a serious possibility too.
Realistically they need a full new team of players with De Gea and maybe Rafael the only ones good enough to play for them. The rest are either aging. already past it or just lost causes.
There won't be a resurgence with this squad, even if there is it will get them Europa League football at best. They absolutely cannot win the league or even finish top 3 now even if they go on title-winning form for the rest of the season.