I hope that they had good games under him, that would be tragic if they didn't. But from Mourinho's perspective they are failure because they didn't fit what he wanted to do and he wasn't able to adapt to what they were consistently good at, it's not that difficult. And I ddin't say that these transfers were dramatic, they are failures for Mourinho because they didn't fit what he wanted which is why Pogba was pushed out of the club and Lindelof not trusted to start.All 3 had good games under him, Lindelof started to come to his own even before Ole. Granted he's been better and better. Pogba has been good also until their relationship fell apart and Matić was one of the best players last season imo.
Sure, Ole has had them more motivated and playing with more sparkle so to say but let's not pretend all of his signings were a disaster. Bailly for instance had problems with injuries but may prove to be a good signing in the long run.