Baltimore Ravens first-round pick Breshad Perriman underwent arthroscopic surgery on his right knee.
"He didn't see anything new," coach John Harbaugh said of the procedure, which was performed by Dr. James Andrews. "There's no new injury of any kind. It's just a slow-healing ligament for whatever reason. It's very slow. They're a little surprised to say the least that it's healing this slowly, but that's what it's doing. So it's just a matter of time."
This continues a frustrating and painful rookie season for Perriman, who has yet to play a snap after being the No. 26 overall pick.
Harbaugh said Perriman got an injection with stem cells.
"It's going to be a little bit of time," Harbaugh said. "Hopefully, shorter rather than longer, and we'll keep working hard to try to get him back."
Perriman is out indefinitely, according to NFL Network, which was the first to report the surgery. This is a major blow to the Ravens, who were depending on Perriman to stretch the field with his speed and replace Torrey Smith, who left in free agency. No Baltimore receiver outside of Steve Smith Sr. has more than six receptions this season.
Perriman had been making progress running before aggravating his sprained knee before Sunday's game against the Bengals. He was trying to catch up to an overthrown deep pass before pulling up in the end zone.
It was only a week ago that Perriman had returned to practice. He had been sidelined eight weeks after injuring his knee on the first day of training camp.
Harbaugh said Tuesday that he was unaware of any setback for Perriman.