As if Cleveland's season couldn't get any worse -- some rando allegedly broke into FirstEnergy Stadium and took a joyride on the field just days before the Browns' next home game ... resulting in a bunch of damage.
The incident reportedly went down early Tuesday morning ... forcing the Browns' grounds crew to rush to repair the grass before Sunday's matchup against Tom Brady and the Tampa Bay Buccaneers.
Pictures captured by News Channel 5 show tire marks throughout the field .