Just watched the post match clip again.
It shows Fellaini attempting to tackle Eriksen from behind. This would have been within a minute or two of him coming on. He manages to get something on the ball but Eriksen then picks up the loose ball. Immediately after that Fellaini starts half-hobbling and reaches down to his left knee. From that point on his mobility seems to be impaired. He goes on to make several half-hearted attempts at tackling /winning the ball from that point on without once making contact so it's clear he needs to come off.
Whether this was all because he did not want to be on the pitch in the first place is another matter, either way Mourinho needed to get him off the pitch pronto. His team were already struggling enough (whahey!) so there was no way he could afford passengers. He had already prepared Rashford but had to send for Herrera instead.
Of course the injury could all have been a charade on the part by Fellaini because he just did not want to be there but as he comes off Mourinho gives him what seems like a genuinely sympathetic arm around the shoulder and says something to him, maybe sanctioning him to go straight to the dressing room, who knows.
Meantime conspiracy theorists will no doubt have a field day.