Steelers Coach Mike Tomlin -- THREATENED by Dolphins Players ... We'll Run His Ass Over

Mike Tomlin just got put on notice by Miami Dolphins players, who tell TMZ Sports ... if the Steeler's coach so much as steps foot on the field this Sunday ... he's gonna need to call 911.

In case you missed it -- Tomlin was fined $100,000 for stepping onto the field last week and interfering with a kick-off return by the Ravens' Jacoby Jones.

TMZ Sports spoke with one of the Dolphins -- who play the Steelers in Pittsburgh this weekend -- and he says ... "I hope he learns his lesson that he needs to get his ass off the field or he might be in a hospital if it occurs again."

Tomlin has apologized and insists the whole thing was an accident. Sounds like he better not make the same mistake twice.