The playmaking safety will miss the remainder of the season with a torn patellar tendon in his right knee after being injured during Sunday's 37-10 victory over the Kansas City Chiefs.
It's the second consecutive year Leonhard has been lost for the season because of a devastating injury. He broke the same leg in a collision with teammate Patrick Turner during practice a little more than a year ago.
"This team means everything to Jim, and his leadership is going to be missed without question," Ryan said. "He's one of the guys that elevates other players around him."
"I want to thank everyone for the support," he wrote on Twitter. "We've been here before and came back stronger than ever. Things happen for a reason, I'll find it."
Leonhard will need to wait for the swelling to subside before having surgery, and then Ryan hopes the safety might be able to be around the team in a player-coach role, as he was at the end of last season. Seven-year pro Brodney Pool will replace Leonhard in the lineup.
Leonhard is one of Ryan's favorites, a player whom the coach brought with him from Baltimore when he took over in New York in 2009. Leonhard is in the final season of his three-year deal with the Jets, so it's uncertain if he has played his last game in New York. Ryan insists Leonhard will rebound from this latest injury, though.
"This is a setback," Ryan said. "This is not going to be the end of Jim Leonhard. I don't think Jim Leonhard is done playing."
The Associated Press contributed to this report.