I know that as a United fan I could be talking out of my ass here, but I would never give that as a penalty for several reasons:
1) Mahrez doesn't actually beat Rojo, the ball is past, but he is not, and he doesn't make an attempt too, instead simply running into Rojo
2) Rojo doesn't make any contact aside from the small kickout, Mahrez is the one who initiates the contact
3) Said kickout is not enough to take out Mahrez, and Rojo pulls out of it anyway
Maybe my homer glasses are turned way on here, but I looked at the play about 20 times and could never see myself giving a pk. In addition, in realtime it looks far more harmless which is probably why no one really appealed it.
This defence of Rojo does not change the fact I think he's an idiot and should be sold in the summer.
Or he has his agenda ... seems way too often !