The Capitals, tied for last place in the NHL, fired head coach Bruce Cassidy on Wednesday and replaced him with Hanlon.
"We are in this spiral this season, and I couldn't sit back any longer," general manager George McPhee said. "We still have every intention of making the playoffs. Ten games under .500 this time of year is dangerous. We can't lose any more ground."
Hanlon was an assist under Cassidy before getting promoted on Wednesday. He immediately promised a patient approach, a stark contrast from the in-your-face style that hindered Cassidy's tenure.
"We're not going to make a lot of system changes, but I think we can make some changes overnight in how we approach the game and how we treat ourselves," Hanlon said.
The Capitals have 18 points, tied with Pittsburgh for fewest in the NHL despite one of the highest payrolls in the league. Washington has struggled with an inexperienced defense and has had trouble scoring despite a lineup that includes Jaromir Jagr, Peter Bondra and Robert Lang.
The Bruins are 1-4-4 in their last nine games. Boston, which is 8-2-2-3 on the road, has tied its last three games.
Playing his third game since being recalled from the minors, Donato scored when he took Sean O'Donnell's long pass at center ice and beat Panthers goalie Roberto Luongo on a breakaway.
"I told our guys, we've got to shoot the puck," Boston coach Mike Sullivan said. "We were looking for the perfect play, and sometimes it's not there."