PITTSBURGH — The Pittsburgh Steelers aren't happy with the way their No. 1 defense has played during the team's first two preseason games, but coach Mike Tomlin feels fortunate the unit will face a stiff test in trying to redeem itself next time out.

The third preseason game is traditionally the one in which the starters play the most, a philosophy reinforced by Tomlin, who said Pittsburgh's starters will play at least a half when the team plays in Minnesota at 8 p.m. Saturday.

The Vikings stout run offense and defense — both ranked No. 1 in the NFL last season — and its Pro Bowlers on the offensive and defensive lines make the team a real challenge.

"There couldn't be a better team to play in the third preseason game than the Vikings," Tomlin said.

Pittsburgh's first-team defense has allowed three long scoring drives during the four possessions it has been on the field in games against Philadelphia and Buffalo earlier this month.

After the Bills game in Toronto, linebacker James Harrison — last season's team most valuable player and a vocal leader — criticized the defense for treating the preseason too casually. Tomlin agreed.

"We didn't have a good showing," Tomlin said. "We want to bounce back."

That won't be easy against the Vikings, who have an offense that includes Pro Bowlers at guard, Steve Hutchinson, and running back Adrian Peterson and another high-profile runner in Chester Taylor.

"We didn't tackle well in the first preseason game and we didn't tackle well in the second preseason game," Tomlin said. "That can't become a trend, and it can't become us. We understand that if we don't tackle well, they have a couple backs in (Nos.) 28 and 29 (Peterson and Taylor) who can go up and down the field on you. We're well aware of that."

The third game will be the preseason contest that most resembles a regular game. In last weeks game in Toronto, the starters played only a series or two.

In addition, this week's game falls on a Saturday and comes after the first week of training at the Steelers home facility. The team broke camp at Saint Vincent College in Latrobe Monday.

"We've got a lot to prove," said the captain of the defense, linebacker James Farrior, on the day his new five-year contract extension was announced.

"We feel like we haven't been playing to our standards the past two games, so we're treating this game like a normal game," Farrior said. "We put that type of preparation in. We're looking forward to going out and seeing what we can do. It'll be almost like a simulated game."

Tomlin confirmed that only one position in the starting lineup is still up for grabs, not including special teams. The position is at center, where offseason signee Justin Hartwig will get his second start in his bid to unseat incumbent Sean Mahan.

Pro Bowl strong safety Troy Polamalu was termed "questionable," but it seems doubtful he will play after just coming off the physically unable to perform list Monday and being limited in practice the following three days.

The special teams units still have jobs up for grabs. Tomlin said each of the two veteran punters in camp to replace the injured Daniel Sepulveda (Paul Ernster and Mitch Berger) will punt for one half.

The punt and kick returning duties still will be split by a number of players vying for those jobs. But on Saturday there will be a twist: Starting receiver Santonio Holmes will be given a chance to return punts for the first time this preseason.