Cahill suffered the injury when he was pushed into goalkeeper Joe Hart during the first half of Saturday's 1-0 win over Belgium at Wembley.
Mertens has at least expressed remorse for his part in the incident, issuing an apology on his Twitter page.
The PSV Eindhoven forward wrote: "I sincerely apologize 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!"