Pietrus' agent, Bill McCandless, said the one-year deal is worth a league-minimum $1.3 million.
Celtics coach Doc Rivers said before Sunday's game that he thought Pietrus might be able to join the team on its current road trip, which features games Tuesday in Miami and Wednesday in New Orleans. Pietrus is scheduled to have his physical Monday in Boston.
Before Rivers spoke, however, public relations director Jeff Twiss, relaying word from general manager Danny Ainge, said that Pietrus is not expected to join the team on the trip and instead is scheduled to make his debut in Boston on Friday night.
"I think he'll be a great addition," Pierce said. "You look at the Eastern Conference with a plethora of great wing players, it's nice to have another guy you can add to the mix who's a pretty good defender and can knock down three-pointers."
"We are very excited to be able to add Mickael to our team," Ainge said in a statement released by the team Saturday. "His skills as a perimeter defender and an outside shooter provide great versatility to the roster."
There could be a health issue with Pietrus. He was traded to Toronto earlier this month, but swelling in his right knee alarmed the Raptors' doctors and the deal was called off. Pietrus had surgery this past summer and missed the last 12 games of the 2010-11 season due to the knee.
After the trade fell through, the Suns worked out a buyout agreement with Pietrus.
Rivers and former teammate Brandon Bass raved about Pietrus on Friday while talking about his potential addition.
"He would be a good addition for any team," Rivers said. "He would be athletic, he would bring some toughness. He can guard 2's and 3's. He can stretch the floor. Whoever gets him will be getting a heck of a player, if he's healthy."
Said Bass: "He's a good defender. I think that's the strength of his game. He can guard the best 2-guards in the league, at least that's what he showed when I was with him in Orlando."
Rivers hinted he wouldn't be opposed to throwing Pietrus right into the fire.
"I told you guys, we don't have any choice this year," he said on Friday. "Usually, if you did add a player, you would practice and show them (the system). But I don't think that anyone who gets a player this year, at any point, off of trades or whatever -- it's going to be throw them in the fire and figure it out. That's the year we're going to have."
Chris Forsberg covers the Celtics for ESPNBoston.com. ESPNBoston.com's Peter May contributed information to this report.