Thing is we've had major squad reshuffle over the last 4 years. I'll play your game here. In process of finding culprits of malaise IMO we can exclude whole pack of central defenders, because they're the least essential group that influences progressive game(+GPs). From fullbacks we get: Valencia who's in form of his life; Darmian - bit part player with probably no future at the club; Blind - fine offensive wise; Young - regular bench warmer with okeyish cameos.
Midfield: Carrick - oldie but still decent, he's not a DM though; Herrera - great, but underused in first season, totally rubbish in second, now asked more often than not to do a mind boggling DM job; Fellaini - the adopted child of 3 successive fathers, he's corrupted & evil, however they've all somehow found endless amounts of love for him. Seriously not good enough in any part.
Attack: it's only Rooney and I think you've precisely had him in mind, cause he fits the diagnosis like a glove; well, there's also Mata - the second striker who's permanently being asked to play as a winger.
For me the blame lies with our managers. Moyes was doing a nothing job, completely bottling transfers windows. LvG did well in the market, yet his downfall came from underestimating PL as well as overestimating his football masterplan aka philosophy. Unwillingness for compromises must be mentioned here.
And last we have our dear Jose. Like I've said he's done well with incoming players, but he should have used winter transfer window for bringing in a DM. Ostracizing (kind of) Shaw and Mkhitaryan while giving Rooney free reigns for few months isn't something to applaud. Delegating Pogba to two-men-midfield, Herrera as a DM. Schneiderlin not getting any chances, from day one it looked like he's not figuring in his gaffer's plans. And Fellaini of course with his magnificent displays.
Our squad is good enough for winning a place in league's top 4. Not achieving that goal will be universally seen as a big failure.