NFL's Brandon Marshall -- I'VE BEEN GROUNDED ... Over Collapsed Lung

Chicago Bears superstar Brandon Marshall says he's on a 4-week timeout from flying -- and can't get on a plane to NYC for a deposition in his assault case ... all because of his collapsed lung.

TMZ Sports has learned ... the wide receiver was scheduled to appear for the depo on December 9th to defend himself against allegations he punched a woman in the face outside of a nightclub back in 2012.

But in court docs obtained by TMZ Sports, Marshall claims there's no way in hell he could've made it to the deposition because doctors have put him on a "no-fly restriction" due to injuries he suffered during a Thursday Night Football game on Dec. 4th.

According to docs submitted in the case ... the WR's overall condition is "good" and doctors had planned to reevaluate the NFL star toward the end of December.

We did a little digging ... and according to the British Lung Foundation, the reduced cabin pressure in planes makes it more difficult to breathe.

For his part, Marshall has denied any wrongdoing -- and has said his wife was the real victim during the night of the incident ... claiming she was hit with a bottle while leaving the club after a scuffle had broken out.