Personally I think Mourinho has bought fine, but his tactics and approach fail at getting the best out of many of these players. Overally his biggest failure IMO is failing to adjust tactics and approach to the talent at his disposal. Pogba, Sanchez, Fred, Lindelof and Bailly are all much more suited to a more aggressive approach where we press higher up the pitch rather than dropping back into our own half out of possession. These are also players, perhaps excluding Bailly, who would thrive in a more attacking setup, probably a big reason that we always look better after falling behind as we are then required to look to control possession and get forward.
Players he's inherited like Martial, Rashford, Herrera, Smalling, Shaw, etc. would also be better suited to a more proactive approach as well IMO. I still don't think the squad has the quality or balance (our right hand side is weak, although Dalot could help if he can get healthy and gets chances) to push for the league or CL, but top-4 with this squad shouldn't be some monumental struggle. There is quality here, but it's not being used in a way to optimize it's efficacy and output.
Whether or not we make a change in season the club needs to get its act together, employ a technical director and start formulating a list of candidates to replace Mourinho if they decide his time is up now or at the end of the campaign. Of course that would require the board not to be run by a footballing idiot like Woodward so that's probably asking for too much.