Gallagher returns from an 18-game absence after suffering a fractured left hand on January 4 in Dallas.
The 24-year-old forward has 6 goals and 12 assists in 39 games this season, and comes back from injury to face the Bruins for a second time in as many seasons.
A year ago, Gallagher returned from a similar injury in a 5-1 win at the Winter Classic.
He will play his 300th career NHL game on Sunday.
Therrien also announced prior to puck drop that Brian Flynn will be back in the lineup in place of Sven Andrighetto, following a two-game stint as a healthy scratch.
Nikita Nesterov likewise returns in place of Greg Pateryn, rounding out the changes in Boston.
