"I don't think it's easy for any of the quarterbacks to pick it up," O'Brien said Thursday on a Big Ten conference call. "It's not the easiest offense in the world to pick up. We have to do a fantastic job of teaching it well and making sure that our players, especially the quarterbacks, understand the emphasis of each play, the concept that we're trying to get done.
"We have to do things a little bit different because we have a different team. We've got veteran tight ends. We've got a receiver [Robinson] that caught [77 passes] last year, and the year before that he only had three. So we've got a veteran team. It doesn't mean necessarily we'll be doing more. It just means that we'll be doing different things."
O'Brien said Bench and Ferguson, who have received equal time with the first team, are "smart guys" who have had some good moments and some bad moments. Without getting into specifics, he said they both "do some things well, and there are some things that both of them need to improve upon."
With only a few practices to go before the annual Blue-White Game that ends spring practice on April 20, the coach said one still could gain an advantage going into the summer.
"Anytime you have practices left, there's always enough time for one guy to separate himself," he said.
O'Brien also praised his running backs - Zwinak, Bill Belton, and newcomer Akeel Lynch, who redshirted last season. He was especially complimentary of Belton, a sophomore from Winslow Township High who began the 2012 season as the starter but saw his playing time decrease.
"Billy Belton, I believe, is a much improved player," O'Brien said. "He's lost some weight, looks quicker. He banged his toe up last week, so he's missed a couple of practices. But he's doing some good things."
Defensively, one of the major concerns is linebacker depth, which O'Brien called "something I think about 24/7." He said he's trying to keep his starters as healthy as possible at every position, making sure players stay on their feet and not dive.
"We don't go to the ground," he said. "We don't want guys diving to make tackles or diving to break up passes. We want guys playing on their feet, moving their feet to get in position to make a tackle . . . to get in position to catch a pass or break up a pass. That's how a high-quality football player practices."
Contact Joe Juliano at firstname.lastname@example.org. Follow on Twitter @joejulesinq.