An extract from FIFA.com:
Serious Foul Play
A player is guilty of serious foul play if he uses excessive force or brutality
against an opponent when challenging for the ball when it is in
play.
Any player who lunges at an opponent in challenging for the ball from
the front, from the side or from behind using one or both legs, with
excessive force and endangering the safety of an opponent is guilty
of serious foul play.
All serious foul play incidents should be worthy of a sending off
even if the culprit doesn't intend on harming or injuring anybody.
So Chelsea fans, does the Kalou incident not match this criteria?