CLEVELAND (Aug. 8, 2006) -- The Cleveland Browns are losing centers at an alarming rate.

Bob Hallen, thrust into the starting lineup when LeCharles Bentley sustained a season-ending knee injury on one of the first 11-on-11 drills early in training camp, has left the club for personal and medical reasons.

"I think they are both serious," coach Romeo Crennel said. "Back injuries are always serious and personal issues make the two equal."

Hallen, who like Bentley is from the Cleveland area, signed with the club in March after spending four seasons with San Diego. He didn't practice Aug. 7, when Crennel said the nine-year veteran had come down with a "back issue."

Crennel did not reveal the nature of Hallen's injury or his personal situation and said it's possible the 31-year-old may retire.

"Bob is dealing with medical and personal issues that he is going to have to work out and get taken care of," Crennel said. "Such to the point where I will investigate the avenues available to me."

Shortly after practice Aug. 8 at training camp in suburban Berea, near Cleveland, the Browns placed Hallen on the exempt/left squad list. To get help at center, the club also traded a conditional pick in next year's draft to the New England Patriots for center Ross Tucker.

The loss of Hallen is another significant blow to the Browns, who open the preseason Aug. 10 in Philadelphia. Bentley, the jewel of the club's free agent signing class, was expected to anchor an improved line and protect second-year quarterback Charlie Frye.

Now, with a month left before the season opener, Crennel has little depth or quality at center.

"I think we know that the center situation is critical," Crennel said. "Two guys who are starters are not here."

Alonzo Ephraim, signed by the club July 30 after Bentley got hurt, is expected to start against the Eagles.

"We'll get a chance to find out if he can play or not," Crennel said. "He does have some NFL experience. He was a free agent coming out of Alabama. There were some redeeming qualities that people liked. He has a big body and I think we'll find out exactly where he stands."

Hallen, who was drafted by Atlanta in 1998, started 47 of 86 career games with the Chargers and Falcons. Crennel noted that Hallen had back "issues" with San Diego but that he had no problems with Hallen's attitude, calling him "a good kid."

Crennel said Hallen could face a fine for leaving the squad but would be welcomed if he decides to come back.

"You have to be open-minded about everything," Crennel said. "You are talking about a starting center. If he's better than what you have on the team, you have to consider that."