ENGLEWOOD, Colo. (AP) -Five-time Pro Bowl center Tom Nalen is headed back to the operating table.
The only player left from the Denver Broncos' Super Bowl teams of the late 1990s, Nalen needs another operation on his left knee after surgery in June failed to put him back on the field without pain.
The 37-year-old Nalen will undergo another arthroscopic surgery on his left knee on Thursday, said coach Mike Shanahan, who is holding out hope that his center will be ready for the season opener at Oakland on Sept. 8.
``It hasn't healed the way we'd like,'' Shanahan said. ``And so tomorrow he's going to go in and get a little cleanup. Hopefully, nothing too serious and relieve some of the swelling and the pain.''
Nalen declined to comment following practice Wednesday.
The 15-year veteran whose 2007 season was cut short by a torn right biceps after five games missed most of training camp before returning to practice on a limited basis last week.
The Broncos and Nalen ruled out another surgery last month when an MRI was negative, but they reversed course this week as the pain and swelling persisted.
``Tom's a guy who's going to play when he's hurt. So for him not to be out there, there's something wrong,'' Shanahan said. ``A lot of times you go in there and do just a little cleanup and it's nothing too serious. If it is serious, at least we've got it done. And we haven't been able to get him back as quickly as we'd like. So, there's something in there. We don't know what it is.''
Nalen, whose five Pro Bowls are the most for a Broncos lineman, has helped Denver to the most rushing yards and the third-fewest sacks in the NFL since 1995.
His slow recovery has made center Casey Wiegmann the Broncos' most important free agent signing.
Wiegmann, entering his 13th NFL season, spent the past seven seasons with AFC West rival Kansas City. He started 111 consecutive games in the regular season, the longest active streak among NFL centers.
In Denver, he's stepped into the starting lineup by default.
``I mean, it's very tough because I know Tommy's a competitor. He wanted to come in and finish off his last year on a good note. And this wasn't the way he wanted to do it,'' Wiegmann said. ``But it's good for me because I get a chance to play and that's what a football player should want to do.''
Not that Wiegmann is penciling himself into the opening day lineup just yet.
``You've seen guys come back from a scope injury in a week, so it's hard to tell,'' Wiegmann said. ``Tommy's a tough guy. He played a game last year on a torn biceps. Most guys can't do that, so he's a football player and he wants to play and he'll be there if he can.''

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