Can is a solid squad player, but he's nothing more than that. He left liverpool because he was unrealistic about his own abilities and thought he deserved a bigger role in a bigger squad (at the time).
His attitude was always good, even the last few months at liverpool, and he works hard, but he isn't really outstanding at any one thing. Henderson and Wijnaldum are not very creative, but they put in the same level of effort, are better athletes, and don't make as many mistakes with the ball. Fabinho has locked down the holding midfield role and can haunt been missed at all.
I think he would marginally improve United, but I dont think he is the type of player you need. In my opinion, his lack of elite qualities mean that he's not really going to fit as a quality starter in any system for a club that competes for major trophies.
If United want a 6, they need someone with better positioning, and if they want an 8, they need someone with more quickness and better decisionmaking.
I do think he would help a good bit with the effort and culture inside the club, but I do think he'd add to United's problem of having a bunch of pieces that don't fit together and function worse as a unit than their talents indicate.