McClain told Matt Zenitz of AL.com he's fully recovered from a hip injury that required surgery in 2016 and is confident he can make an instant impact if signed.
"Every time I got in my linebacker position, it was bone-on-bone grind," he said. "Not only do I feel good, I'm recovered and I'm rested. I feel better now because I know I'll be playing with two legs."
McClain last played in the NFL for the Cowboys during the 2015 season. He signed a one-year contract extension to remain with Dallas for 2016. However, he was suspended for the first 10 games of the season and then suspended indefinitely for violating the league's policy on substances of abuse.
The 29-year-old University of Alabama product told Zenitz he's been in contact with the NFL Players Association about reinstatement and wants a chance to write a new ending to his football career.
"I'm nowhere near satisfied with how my professional career has gone," McClain said. "That's for myself. That's me looking in the mirror and talking to my boys. Nah, that's not the taste I want to leave in my mouth."
The Raiders drafted McClain No. 8 overall in 2010. After three years in Oakland, he signed with the Baltimore Ravens in April 2013 but announced his retirement one month later.
The Alabama native was expected to return for the Ravens in 2014 but changed his mind and remained retired.
"I gotta follow my heart. It ain't football," he told ESPN's Seth Wickersham at the time. "If football made me complete I would play. But whenever I think of it my heart pulls me away from whatever reason. ... This means I'm done."
However, after Baltimore traded him to the Cowboys in July 2014, McClain did return to the gridiron and played two seasons in Dallas.
He recorded 412 total tackles, 26 passes defended, 9.5 sacks, four interceptions and two forced fumbles across 65 regular-season games.
If signed, McClain will likely need a strong performance during training camp and the preseason to secure a spot on a team's final 53-man roster.