Robitaille, signed as a free agent earlier this season, scored twice to stake Ottawa to a 2-0 lead early in the second period, but was hit by Thrashers defenseman Garnet Exelby and didn’t return.
A team spokesman said he suffered facial contusions and it wasn’t known if he would miss any more time.
The Senators (10-1-0) almost blew it without him. Although Ottawa scored three more times, the Thrashers rallied with four unanswered goals in the third period, including three by Ilya Kovalchuk.
Daniel Alfredsson scored twice, once into an empty net, and set up two others and rookie Nick Foligno and Patrick Eaves also had goals for Ottawa, which needed just 15 saves from goalie Ray Emery, who was making only his second start of the season.
Dany Heatley and Antoine Vermette each added a pair of assists.
The Senators chased Thrashers starter Johan Hedberg after scoring four times on 19 shots. Hedberg was replaced by Ondrej Pavelec.
Atlanta (4-9-0), playing the sixth game of a seven-game road trip, lost for the first time in three games.
Eric Perrin, who also picked up an assist, had the other goal for the Thrashers, who struggled for most of the night to get any offense going before getting their four goals in the third period. They managed just four shots on Emery in the first period and only two in the second.
Slava Kozlov chipped in with two assists.
The Senators were playing without Jason Spezza, who suffered a groin injury in practice and is listed as day-to-day.
Robitaille was playing on the Senators’ top line along with Alfredsson and Heatley as a result of the injury to Spezza.
It didn’t take him long to make the most of the opportunity.
At the 1:48 mark of the first period, the journeyman center, signed as a free agent on Oct. 16, took a pass from Alfredsson and beat Hedberg with a low shot for his first goal as a Senator and first since March 24, while with the New York Islanders.
The Senators then scored two goals on their first two shots of the second period, just 50 seconds apart.
Robitaille took a pass in the slot from Alfredsson and beat Hedberg for his second goal and Eaves snapped a shot from the slot that beat the goaltender to the short side to make it 3-0.
That would be among Robitaille’s final contribution to the game, however. He was run over in the slot by Exelby and had to be helped to the dressing room after appearing to bang his head on the ice when he fell.
No penalty was called on the play.
Before the second period was over, Alfredsson beat Pavelec after being set up by Heatley and Ottawa led 5-0.
Kovalchuk spoiled Emery’s shutout bid midway through the third and started Atlanta’s rally when he snapped a shot between the goaltender’s pads on a 2-on-1 break.
Ottawa’s Chris Neil took a four-minute roughing penalty late in the game and Kovalchuk notched his second and third goals on a pair of hard shots to make it an interesting finish.
Notes: The game was the 400th of Kovalchuk’s career. ... The Thrashers wrap up their road trip Saturday night at Tampa against the Lightning. ... The Senators play a home-and-home series against the Boston Bruins on Saturday and Sunday, respectively.