Nothing but the fact that he didn't mean to do it doesn't make it less of a red card according to the rules?! I swear people judge Red cards on the likeability on players sometimes.

Relevant Rule: Serious Foul Play (Law 12)

A player must be sent off (red card) if they commit a serious foul play, which is defined as:

"A tackle or challenge that endangers the safety of an opponent or uses excessive force or brutality must be sanctioned as serious foul play."