Eddie Nketiah was red-carded shortly after replacing Alexandre Lacazette following an intervention by VAR.
The referee initially gave a yellow card for what looked like an accidental coming together, however, video replays did show that it was serious foul play.
In all fairness to Nketiah, the ref, who had a straight view of the incident must have thought it was a genuine attempt to get the ball, that is what it looked like to me and that is why he got a yellow.
But in slow motion on video, it did look a bit rough.