I think we need to disregard the lack of rotation issue here. This is very much an atypical run-in.
Even so, both McTominay and Fred started the game against Spurs, and both have had two further starts since the restart, which accounts for around a quarter of the matches we've played thus far - a reasonable amount considering they are squad players.
Like you say, Ole has in the past been able to rotate players, it's just that the situation with the Top 4 race, and the squad players lack of sharpness and form, has meant he hasn't had much chance to. I think he has tried to rotate, and tbf to him, he did sub off the likes of Bruno, Pogba, Martial, Rashford et al after c.60 mins in all the games we've had until the Palace match. The problem has been that the players he's replaced them with just haven't been able to keep things ticking over to the extent that when it came round to the FA Cup SF, he was caught between a rock and a hard place.
He tried to rotate en masse against Norwich as well, and they almost fecked it up for him and he's been spooked ever since. This is not to say that the squad players are shit and need replacing. They're not. Some of them are damn good players who have played a crucial role in getting us anywhere near Top 4, but the issue is that they have come back from a 3 month layoff lacking in sharpness and form, and with our situation vis-a-vis Top 4, we simply were never in a position to allow for dropped points in those initial games.
I do agree though, that he should have looked at changing the template of the midfield, with Pogba pushed further up in place of Bruno from time to time.