It was originally feared that the injury, picked up in the 0-0 draw with Brentford, could end Merson's glittering career, but the creative 37-year-old midfielder isn't finished just yet.
He said: "The main thing is that it's not ruptured.
"That was my initial fear as I was in a lot of pain and something like that would have ended my career.
"I am waiting on the results of a scan to determine the extent of the damage and to see how long I will be out."
Merson put in an outsatnding performance as his Walsall side beat United 3-1 last week.