The Vikings began full-squad training camp Friday with five players sidelined, including two key defenders in cornerback Mike Hughes and defensive tackle Shamar Stephen.

Hughes was among three players placed on the Physically Unable to Perform (PUP) list. He will count against the 90-man roster limit, but will not be able to practice with the team until medically cleared.

Head coach Mike Zimmer revealed Friday that Hughes is actually recovering from a "multi-ligament" knee injury, not just the ACL tear Zimmer said Hughes suffered Oct. 14 against Arizona.

"We'll see how he's doing," Zimmer said. "He's ahead of schedule, he's doing great. So we just have to see where it goes. Some of them will be short term and we'll just have to see where it goes."

Hughes' recovery will test the Vikings' cornerback depth, along with Holton Hill's eight-game suspension for performance-enhancing drugs and substance abuse.

"Mike works really hard," Zimmer said. " [Vikings head physician Chris] Larson, who did the surgery, is excited about where he's at right now. He's just not ready to practice yet."

Zimmer did not elaborate on Hughes' knee damage.

"I'm not smart enough to know all those things," Zimmer said. "I know he had an ACL and then he had something else."

Tight end David Morgan and receiver Brandon Zylstra were also placed on the PUP list to start camp. Zylstra is dealing with a left hamstring issue, according to a source. Morgan, the blocking tight end who was unable to practice this spring, missed three games at the end of last season due to a knee injury.

Defensive linemen Shamar Stephen and Tashawn Bower were placed on the active/non-football injury list.

Stephen, who is expected to start next to Linval Joseph, isn't expected to be out long, according to Zimmer. However, Bower tore his Achilles tendon while working out on his own.

Because Bower was placed on the non-football injury list, the Vikings can choose whether to pay him. Zimmer said the Vikings "treated him the right way."

"We didn't take money away from him," Zimmer said. "He's been working real hard. He's really improved throughout the offseason."