At this point, it's unclear what kind of injury or issue Dareus is dealing with, but he'll be unable to practice with his team until he's moved to the active roster -- which could happen at any time.
Dareus, coming off his best season in 2013 with 7.5 sacks and his first Pro Bowl election, has had a rough offseason. In May, he was arrested and charged with possession of a controlled substance and possession of drug paraphernalia after police said they found synthetic cannabis on Dareus after pulling him over for speeding.
In June, he was charged with three crimes (reckless endangerment in the second degree, reckless driving and participating in an illegal speed contest), and he didn't attend the rest of the team's OTAs.
In April, the Bills exercised their fifth-year option on Dareus, meaning he's tied to the team through at least the 2015 season.