Fortson underwent successful surgery Thursday to repair a stress fracture in his right foot and is expected to be out 8-12 weeks, Celtics spokesman Jeff Twiss said.
Conlon, 31, played the last two seasons with the Miami Heat. He appeared in only seven games last season because of hand and finger injuries.
Conlon, a 6-foot-11, 235-pound forward, played for the Celtics in 1996-97, averaging 7.8 points and 4.4 rebounds in 74 games. He averaged 22.7 minutes per game during that span.
The Bronxville, N.Y., native has played for seven NBA teams during an eight-year career, averaging 6.5 points and 3.5 rebounds in 376 games.
Before turning pro, he played four seasons at Providence College, where he finished as the 18th leading scorer in school history with 1,135 points.
Fortson, a 6-foot-7 foot, 260-pounder, averaged 11 1/2 rebounds and seven points in two preseason games since coming to the Celtics in a multiplayer trade with Denver in August.
Doctors inserted a pin into the injured bone in his foot during surgery at New England Baptist Hospital on Thursday.
Team officials don't know how Fortson, 23, hurt his foot, but doctors ordered X-rays Tuesday when he complained that nagging pain was getting worse. The tests revealed a stress fracture.
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