Both returns come as something of a surprise, as it was being reported as recently as this morning that Mendenhall would begin the season on the PUP.
Mendenhall and Hampton are both recovering from ACL tears that they suffered toward the end of last season. Mendenhall's injury occurred on January 1 against the Browns, and Hampton's was a week later during a playoff loss in Denver. Just yesterday, Sports Illustrated guessed that Mendenhall would likely miss the first six weeks of the season.
Both players had begun light workouts, but no indication had been given that either was close to returning. They are both in the final year of their respective contracts.