Merson said: "If there is a rupture it could be the end of my playing career.
"I heard a pop and I was in a lot of pain but it has eased off now and hopefully it's just a strain."
The Saddlers, who beat Scunthorpe 3-1 on Saturday, moved up to seventh in the table after the result.
Merson added: "I thought the lads were outstanding, not in football terms but in battling for a result, especially after losing at home to Chesterfield two weeks ago.
"This was probably the best result since I have been manager. You have to match Brentford for work-rate, and we did that."