Willie Parker admits he was shocked at Mike Tomlin‘s decision to step away from the Pittsburgh Steelers after 19 seasons … but tells TMZ Sports he has no doubt the Super Bowl-winning head coach will once again roam an NFL sideline!

We spoke to the former Steelers running back star shortly after Tomlin made the tough decision to resign … bringing an end to his reign as the NFL’s longest-tenured coach after nearly two decades without a losing season.

“Man, it was surprising,” Parker told us on Tuesday.

“I never would have thought that, you know, he would be stepping down right now as a Steelers head coach.”

Parker called Tomlin one of the National Football League’s greatest coaches and said, despite all the criticism and chatter from this past season, MT has earned his flowers.

That said, Parker — who ran for 5,378 yards and 24 rushing touchdowns in a six-year NFL career (three under Tomlin) — believes Tomlin will coach again when he’s ready.

“Who wouldn’t want a winner in the locker room?” Parker asked.

“I can understand if he wanna just chill, but I can also understand if he wanna just keep going, keep coaching and taking it even further — taking his legacy even further.”

As if this coaching cycle wasn’t already crazy enough …