To make things worse ... Saturday afternoon -- in only his 2nd game QBing for the AAF's Memphis Express -- Manziel sustained a head injury while tackling former NY Giants player Will Hill ... after he picked off a Johnny pass.
Manziel laid injured on the field ... and had to be helped off. He didn't return to the game.
The team hasn't said if Manziel's head injury will force him to miss any time (they have a game this Saturday in San Antonio) ... but the fact that he went to a nightclub probably indicates the injury ain't too serious.
That would be some good news ... 'cause sources close to Johnny tell us he's immersed himself in his football career following his breakup -- with the end goal being a return to the NFL.
Fun fact, the jacket Johnny's rockin' -- the Lovers baseball jacket by AMIRI -- retails at around $2,500 and has an image of a crumbling heart on the back.