Cardinals safety Tyrann Mathieu watches on the sideline as teammates go through the conditioning test Thursday.

If he had to – if Bruce Arians really felt it necessary – Tyrann Mathieu thinks he could be active when training camp practice begins Friday, at least to do individual drills and run around.

But less than eight months after tearing his right ACL, Arians doesn't think it's necessary, and Mathieu doesn't plan on getting out on the field. Not yet.

"I want to take my time with it," said Mathieu, currently on the active physically unable to perform list. "If I had to guess, I'd be

back in three or so weeks."

Mathieu said he is optimistic about the future and continues to be hopeful he will be ready for the regular-season opener against the Patriots. But after his left knee was shredded with multiple ligament tears in 2013 – leading to a subpar 2014 – "I want to be confident that I am who I am," Mathieu said. "I don't want to go out there and put anything on tape that's not reflective of my type of player."

The Cardinals have a few key players on the sideline as practice begins Friday. Cornerback Justin Bethel (foot) is also on PUP after Arians said he suffered a setback about three weeks ago. Pain tolerance will be the factor to when Bethel returns, Arians added.

First-round pick Robert Nkemdiche suffered an ankle sprain during team work on the first day of QB School Tuesday, and is day-to-day for now.

Arians said there is no reason to rush back defensive tackle Frostee Rucker (foot), who is also on the PUP list.

The coach said the same about Mathieu, at the same time praising the safety for his work ethic. Arians did express concern about Mathieu playing with a knee brace, which is what Mathieu had to do in 2014 coming off his previous injury.

"He's going to struggle again with that brace on just like he did that year," Arians said. "But he knows how to do it now and he's used to it."

Mathieu reiterated how 2014 wasn't a good year personally. But he said that, other than the knee situation he's in the best shape of his life, and that going through one ACL tear and comeback will only continue to help him this time.

Mathieu also said he remains optimistic about a potential contract extension with the team – he is going into the last year of his contract – although he's trying to focus only on getting healthy.

He admitted that not thinking about the contract can be difficult.

"Obviously," Mathieu said, chuckling. "It's a lot of money."