The attacker was chasing down a long ball, and gave the centre-back a shove in the back which meant he collided with goalkeeper Joe Hart, taking the full force of the Manchester City man's shoulder to his jaw.
Speaking via his Twitter account, Mertens said: "I sincerely apologize [sic] for the harm I've cost to Cahill. When in the heat of the moment, you often don't think about the consequences.
"My thoughts go out to him and I hope he recovers well!”
Liverpool youngster Martin Kelly has been called up by Roy Hodgson to replace the injured Cahill.