United's defence need sorting out for sure, because what we had on the right side this season was utter crap, but there was a time under Mourinho when I thought Eric Bailly had the potential to be world class. Tuanzebe might also be good enough. Consistency is really key here. I'd be really happy with Koulibaly, he does look legit.
BUT...
Our attack is of significantly greater concern to me. And it worries me MASSIVELY that we don't have players who can stick the ball in the back of the net, also, our midfield is pretty shambolic.
It's true that a more comfortable back 4 will allow more freedom for our attack, but it could also be argued that a good attack can hide defensive frailties by outscoring the opponent. I feel SAF always placed slightly (~significantly, at time) more emphasis on his attack and midfield. I don't recall SAF ever building the team from the back.
Yeah, I'm kind of echoing what some on here are already saying regarding adding the fairy dust after we build a solid team of grafters. But it's slightly different in that I'm saying we should make do with our defence, and focus first a foremost on our problem of scoring goals. This defensive mindset is a holdover from Mourinho's time I think. In fairness, Smalling, Bailly, Jones, tuanzebe is good enough to challenge for top 4 this season. In all honesty, we are stacked in centre defence. True, the quality isn't quite there, but United aren't challenging for the title next season.
Most pertinently, Young needs replacing with a world class wing back. Someone who can contribute to the attack.
A right winger... WE HAVE NO RIGHT WING WHATSOEVER.
We need a DECENT STRIKER. Even Cavani for a decent two years, just anyone who could contribute 20+ goals per season, and for the love of God, can hold the ball up and has a decent first touch.
The midfield, we obviously need reinforcements after Herrera.
Seeing as we didn't sign anyone in the last transfer window, maybe the kitty rolled over into this window. So if it's 200m + koulibaly, then by all means, United should sign koulibaly, but if Ole has to blow almost half the budget on a defender now, I would say: "No way"... and ask yourself: "where are the goals coming from?"