Cardinals defensive backs Justin Bethel (left) and Tyrann Mathieu (right), here celebrating a Mathieu interception last season, will be two of the four players who will start training camp on the PUP list.

The Cardinals will have four players on the active physically unable to perform list to start training camp.

Safety Tyrann Mathieu (knee), cornerback Justin Bethel (foot), defensive tackle Frostee Rucker (foot) and linebacker Zack Wagenmann (foot) are all expected to be on the sideline at the outset of practices, which begin Friday.

The team began some work Tuesday at University of Phoenix Stadium, with the first of two "QB School" sessions with rookies, quarterbacks and a handful of players returning from injury.

Coach Bruce Arians had previously said Mathieu and Rucker likely weren't going to be ready for the beginning of camp. Bethel had been hopeful he'd be ready. All four can return at any point in camp once they are cleared to play.

The absence of Bethel further throws into flux the cornerback situation. The veteran should be the favorite to win the starting job opposite Patrick Peterson, but he missed all of the offseason with his broken foot and now must wait to get on the field.

The Cardinals recently signed veteran Mike Jenkins to compete for the job and had fellow veteran cornerback Chris Culliver – who himself is coming off an ACL injury – in for a visit this week. The team also drafted Brandon Williams and Harlan Miller at the position.

Mathieu is ahead of the rehab pace he set back in 2014 coming off a much more serious knee injury, and the safety – who also plays a lot of slot cornerback – was brought off of the PUP list midway through camp that season.