He didn't lose his leg in the Sarlaac. It had just started to digest it a little, so later on, it probably got very infected or had severe muscle damage. That's why he did the bounty on that Devaronian guy, The Butcher of Montellian Serat. That way, he could pay to have a new leg cloned for him. However, when he returned to Kamino to have his bad leg replaced, the Kaminoan cloner he saw—his old associate, Taun We—remembering the Mandalorian attacks on Kamino during the Clone Wars, asked Fett to hunt down the last surviving Mandalorian responsible. Fett hunted down Fenn Shysa (Mandalore, at the time) on the planet of Shogun. The details of the encounter are unknown, but Shysa met his death—not at Fett's hands, but, ironically, through saving Fett from an unknown threat.
--Sadriel Fett (BFFC Moderator)
"I'm just a simple Fan, trying to make my way in the universe."