Play video content EXCLUSIVE TMZSports.com

If Mitch Trubisky isn't the Chicago Bears starting QB in Week 1, "You've got problems" -- so says Bears legend Mike Ditka.

To say Da Coach was impressed with the rookie's preseason debut Thursday night is a gross understatement ... Ditka believes it's TRUB OR CONSEQUENCES!

"What I saw out of Trubiksy for a rookie in his first game I thought was outstanding."

Trub -- the #2 pick in the draft -- went 18-25 for 166 yards and a TD after replacing starting QB Mike Glennon who went 2-8 for 20 yards and an interception.

Ditka thinks the decision is obvious ... and says Mitch made a better first impression than guys like Tom Brady and Fran Tarkenton.