Frank spent six-plus seasons as coach of the Nets before a 0-16 start to the 2009-10 season cost him his job. New Jersey advanced to the playoffs in each of his first four seasons, losing in the conference semifinals three times. He boasts a career record of 255-241.
Frank joined NBA TV last month as an analyst, but speculation that he could be on Boston's radar intensified after he was shown sitting next to Celtics coach Doc Rivers during the AirTran Summer League earlier this month in Orlando.
"I think it's a good situation for me -- I wanted to get back and coach, and I didn't want to sit and wait, and you can't ask for a better situation or a better organization," Frank told the Star-Ledger.
And Frank says he feels he's walked into a team that remains a championship contender.
"No, the window is definitely still open," Frank told the Ledger. "This team showed its commitment by re-signing Paul [Pierce], Kevin Garnett got stronger as the season went on last year, Ray Allen obviously has plenty left in the tank, and Rajon Rondo is an emerging superstar. Now we just need [Kendrick] Perkins to get healthy."
Frank replaces Thibodeau, who served as an assistant and associate head coach for three seasons and was regarded as Boston's defensive mastermind. Thibodeau last month accepted an offer to become the head coach of the Chicago Bulls.
Chris Forsberg is the Celtics reporter for ESPNBoston.com. Follow him on Twitter.