PITTSBURGH — Sometimes, one player really does make that much of a difference.

The Pittsburgh Steelers' defense wasn't the same last season with safety Troy Polamalu limited to three games by two knee injuries. It wasn't a coincidence they began a five-game losing streak immediately after he was hurt for the second time.

The pass defense wasn't as good without Polamalu. The rush defense wasn't as good. The secondary's confidence level wasn't the same as the Steelers finished 9-7 and missed the playoffs.

"I don't think one man makes that big of a difference. There's not one player who is irreplaceable," Polamalu said.

His teammates would disagree. To the Steelers, Polamalu essentially is that — irreplaceable.

That's why the Steelers were pleasantly surprised when the five-time Pro Bowl player altered his usual offseason training regimen by reporting to voluntary offseason practices this week. Normally, he trains in California with fitness expert Marv Marinovich during the spring.

"Last year was disappointing, so it's good to get out there and see how the knee feels in a football-type environment," Polamalu said. "You can never really simulate team 7-on-7 drills or team drills."

Just as the Steelers found it difficult to duplicate their success without Polamalu.

With Polamalu making seven interceptions on a Super Bowl-winning team, the Steelers were No. 1 in total defense, pass defense, points allowed and No. 2 in rushing defense during the 2008 season. Last season, they slipped to No. 5 overall, No. 12 in points allowed, No. 16 against the pass and No. 3 against the run.

"It affected what we could call as a defense, and it was big in what offenses tried to do against us," safety Ryan Clark said Tuesday. "He's a guy you have to account for. He's a guy who can erase some bad plays. And, obviously, you miss the big plays."

Except the ones they gave up themselves. The Steelers allowed 26 passes of 25 yards or more, or nearly twice as many as the league-low 14 they permitted the season before, when Polamalu played every game.

One more statistic: When Polamalu played beyond the opening series of a game last season, the Steelers were 4-0. When he didn't, they were 5-7.

"You take away what he brings to a team, that means a lot," Clark said.

Polamalu's absence due to medial collateral ligament and posterior cruciate ligament injuries in his left knee may have led to some of those big plays because the other Steelers defensive backs were adjusting to being without him.

Sometimes, they didn't.

"Guys were rolling in different packages that I'm not used to playing with, not used to communicating with," Clark said. "We missed that across the board, not being able to communicate silently. We couldn't pass (signals) on down the line because we're used to playing with the same four, same five people every week."

Clark, a free agent, re-signed with Pittsburgh in part so he could continue playing with Polamalu rather than with a less-skilled safety on another team.

Polamalu hasn't committed to taking part in all of the Steelers' offseason practices; he believes the conditioning work he does with Marinovich, the father of former Southern Cal quarterback Todd Marinovich, is beneficial.

But Polamalu missed 19 full games to injury the last four seasons, five in 2007 and 11 last year, and all but a few plays of several more. That may be one reason why is spending some of his time working out in Pittsburgh.

"I'm rediscovering old things," Polamalu said.

For now, he's not wearing the knee brace he was supposed to wear until the preseason. Polamalu doesn't like how it restricts his motion, and he felt like himself again as soon as he discarded it.

"He looks good," coach Mike Tomlin said. "It's good to have him back out there with his teammates. He's excited to do it."