Let's see...
Rashford: promoted by Van Gaal, became truly good under Ole.
Martial: signed by Van Gaal and has been inconsistent and injury-prone for most of his time here. His best season came under Ole.
Shaw: signed by Van Gaal, benched by Mourinho, regained his form under Ole.
McTominay and Lingard: never good enough to start for United.
Greenwood: Ole gave him his professional debut, not Mourinho. And he was one of our best and most promising attackers when Ole got sacked.
Pogba, Dalot and Lindelöf have already been discussed.
Honestly, Ole left behind a better team than Mourinho did. He did a worse job, but the team was better. Bruno, Varane, Rashford (improved), Shaw (improved), Greenwood (developed), De Gea (not as bad as in 2018) and AWB (better than Valencia/Dalot). It's still nowhere near good enough of course, but it's better than Pogba, Herrera, undeveloped Rashford and benched Shaw.
Rashford: Was at his best in Ole's first full season, then got respectively worse each season.
Martial: Was at his best in Ole's first full season, then got respectively worse each season.
Greenwood: Had his breakthrough in Ole's first full season, then got respectively worse each season.
Pogba: Had his last decent season in Ole's first full season, then got respectively worse each season.
Shaw: Breaks the mold by being at his best in Ole's second season, then followed the pattern by getting worse anyway.
De Gea: Had his best ever season 6 months earlier, and although he'd dropped off a bit at the time Ole took over nobody was too worried. Wasn't until a few months after Ole took charge that his form absolutely plummeted and he was downright poor for most of the time since.
McTominay: Had the occasional period where he showed some potential, but never developed whatsoever. His best patch was late 18/19 and early 19/20 (so once again just after Ole took over) before dropping off and absolutely stagnating since.
Lingard: Obviously not a starter, but was decent squad-player level for LVG, Mourinho and Ole's first half a season. Went to shit in Ole's first full season.
Dalot: Talented young fullback that never showed any development until after Ole left.
At the point Ole took over, not many people wanted many (or even any) of these players sold, whereas by the time he left it was a very different story. They'd all gone from being fairly young players with promise and potential that everyone was hopeful could be developed, to now being around the middle of their career with most of them considered downright failures. In pure ability, the post-Mourinho vs post-Ole teams were fairly even. But the former had a hell of a lot more potential to grow and develop.
Of course, all this ignores my original main point that it was the mental aspect that was the biggest difference between them. It was 'only' bad morale at the end of Mourinho, which could be recovered from relatively easily. Whereas at the end of Ole the mental aspect was in a much worse situation with no easy way back.