The Raiders rank 31st in rushing and are struggling to master new offensive coordinator Greg Knapp’s zone-blocking scheme. They rushed for just 23 yards in a 35-13 loss last week to Miami when Darren McFadden was held to 22 yards on 11 carries.

So when Knapp took the podium Wednesday for his weekly press session, he all but pleaded for patience.

“Let’s keep things in perspective for a second here,” Knapp said. “We’ve played two games in the regular season on offense. We’ve had two different centers and a right tackle who played half a game last week. We need some patience.

"You can’t develop a scheme in two weeks in a regular season, especially when last week we faced the No. 3 ranked rush defense from 2011 and the week before they held the No. 2 rush offense in the league to 2.4 yards per carry, the Houston Texans. That was a good team we faced last week. The scheme will be fine. It just takes some time.”

Knapp said patience is the same message he’s preaching to his players.

“We want success right away but we also have to understand that it takes a little time to develop and (when) we change the parts on them like we have at the position of center, most importantly, and the right tackle spot, it’s going to take a little more time. I’m very confident we’ll be fine. I say that because we had some success in the preseason running the ball effective. We had success the first half of the San Diego game running the ball. We just didn’t last week.”

In a radio interview on Sirius XM, former Raiders offensive line coach Bob Wylie said he thought McFadden wasn’t a good fit for zone blocking schemes because he’s “so fast he outruns the zone block."

“I have to speak to my experiences,” Knapp said. “The guy who was in Houston (Arian Foster) was a pretty fast runner as well, and he fit in fine with the zone scheme. He also was a young back with an offensive line that had run the scheme for five years, and he reaped the benefits of having a line that knew all the nuances.

"Justin Fargas, when we were here in (20)08, ran for 1,000 yards, and he had a lot of speed, too. Darren McFadden, I don’t care what offense you put him in. He’s going to be fine. He’s a good running back in all schemes. We’ll be fine.”

Injury updates: Starting middle linebacker Rolando McClain (concussion/ankle) returned to practice Thursday after missing practice Wednesday. So did reserve cornerback Coye Francies (concussion). Both of them still need to be cleared by doctors to play Sunday against Pittsburgh.

Starting right tackle Khalif Barnes (groin) and starting right cornerback Shawntae Spencer (foot) missed practice again. Willie Smith will start at right tackle if Barnes can’t play. Raiders coach Dennis Allen said he hasn’t decided who would start if Spencer is out. The Raiders lost starting left cornerback Ron Bartell (broken shoulder blade) in the opener.

Free safety Michael Huff took some more snaps at cornerback Thursday, and Allen said Monday that moving him to corner is an option.

Extra points: It’s looking more and more likely that the Raiders won’t have to face Steelers SS Troy Polamalu (calf) or OLB James Harrison (knee). Both of them missed last week’s game and both missed practice again Thursday. “They got very capable backups,” Allen said. “They played well last week. So it’s going to be a challenge to face their defense, regardless of whether those two guys play or not.”

Follow Raiders reporter Eric Gilmore on Twitter: @CBSRaiders.