Starting to think we just need 15-16 genuinely class players to compete with the Big 3 in Europe. Then youngsters and committed, loyal and versatile players like Valencia, Jones and Young as depth rounding out the squad.
16 players (11 first choice + 2 attackers, 2 midfielders, one CB, one fullback and one more player, normally a midfielder of some kind depending on your manager's wants).
Right now, I'd say we have the following players who are good enough or will more likely than not become good enough to be part of that 16 to compete with the Big 3 in Europe, assuming we don't count Carrick due to age and Rooney for obvious reasons.
I'm giving Darmian and Memphis the benefit of the doubt because they have clear strengths, are starters for the Dutch and Italians, are probably just experiencing growing pains after their first real move (Darmian had a few games outside of Torino early on). Blind gets the benefit of the doubt because he's basically been a 7/10 everywhere he's been used and is still just 25 and gaining experience at the 3 positions he plays.
Attack (3) - Martial, Memphis, Mata
Midfield (3) - Schweinsteiger, Herrera, Schneiderlin
Defense (4) - Shaw, Smalling, Blind, Darmian
Keeper - De Gea
That's 11.
And for the other 5, hopefully one of Januzaj/Wilson/Pereira emerges as a serious attacking option. That would leave us to add another attacker, another CM and a CB to have a complete squad.
At that point, upgrades can be considered depending on who's performing. But we basically need the summer Citeh just had in signing 3 stars with multiple years left in Sterling, De Bruyne and Otamendi, except one of those 2 attackers should be a box to box midfielder to allow us to go 4-3-3 more, and because we don't have anyone likely coming through in central midfield but we do have those 3 young attackers with serious promise.
TLDR: Griezmann, Gundogan and a top CB and we'll be really, really good.