Rich suffered the injury at training on Wednesday when completing a contested ball drill.
AFL.com.au was there to witness the seemingly innocuous incident that left the 22-year-old crumpled in a heap on the ground.
Rich tried to tackle Patrick Karnezis, but appeared to collide with another teammate, and fell awkwardly.
He lay on the ground clutching his left shoulder and was then assisted from the field by Lions staff holding his wrist to support the shoulder.
The Lions are expected to confirm on Thursday morning that Rich has a fracture in the shoulder and will miss up to two months.
It is the last thing the Lions need ahead of Sunday's crucial match against Melbourne at the Gabba.
The blonde midfielder won the Michael Tuck Medal as best afield in the NAB Cup premiership, and despite a slow first month, is a barometer of Lions' success.
Opponents have identified him as the Lions' most influential player and sent taggers to him in each of the first four rounds.
The club is already on the back foot with a 1-3 start that has heaped more pressure on out-of-contract coach Michael Voss.
Aside from Rich they will also be without suspended winger Pearce Hanley, while Ash McGrath (calf) faces a fitness test.
Simon Black (knee) is playing his second reserves match of the season on Thursday against the Gold Coast reserves in Mackay and is still at least another week away from returning.
Michael Whiting covers Brisbane Lions news for AFL.com.au. Follow him on Twitter: @AFL_mikewhiting