Last season, Clay had his knee drained at least a half-dozen times, a league source tells ESPN, and one reason the Dolphins balked at matching the offer sheet that Clay signed with Buffalo were the concerns about his longevity.
There were some who wondered how much longer Clay, 26, would be able to play. One doctor predicted that Clay would not be able to have much beyond one productive season in Buffalo, especially considering that he will be playing on turf at Ralph Wilson Stadium.
ESPN NFL Insider Adam Caplan reported last month that teams were concerned about Clay's knees when he was a free agent this past offseason.
Clay's knee already began acting up this summer in training camp, forcing Buffalo to rest him. Clay's knee issue cropped up during an Aug. 25 practice. He played in the Bills' preseason game Aug. 29, catching a 67-yard touchdown pass against the Steelers, but then was rested during the final week of the preseason.
"I'm fine," Clay said after leaving the Aug. 25 practice. "It was just something that they wanted to look out for me a little bit. It was something that started nagging, so they decided it would be best to pull me out."
Asked if it was the same knee that gave him problems last season for Miami, Clay said, "I don't want to get much into all of that. You can ask [Coach] Rex [Ryan]. I don't want to get too much into injury talk. But I'm 100 percent fine, so I'm not worried though."
The South Florida Sun Sentinel reported last December that the Dolphins "limited the practice participation of the team's starting tight end all season trying to get him to game day," but that "the stress of Clay's knee injury eventually strained the hamstring in his right leg."
Clay's contract with the Bills included $24.5 million in guaranteed money.
ESPN.com's Mike Rodak contributed to this report.