IMO when Fellaini lowers his head toward SA, he bends down over him and stops short of making contact, SA then moves his head into Fellaini and then falls back.
Can see why the Ref gave it the way he did, but probably a more accurate reflection of the incident would have been a yellow card each (for stupidity on both counts), which would have resulted in Fellaini going off anyway.
Thought he was also very lax on some of the treatment of Rashford, constantly blocked making runs and tackled from behind with no retribution.