There are arguments both ways as to whether Bellerin was fouled, however one thing is

[PSRB]

I'm not so sure - Alonso looked only at the ball and his arms weren't in an unusual position for someone that was jumping for the ball.

If we had a goal disallowed for that I'd be pretty irritated.

I would suggest that if you knock someone unconcious, by default that's dangerous play and therefore a foul and a card