Forsberg was taken off the ice on a stretcher at 14:37 of the third period in a 6-3 loss to the Edmonton Oilers on Saturday following a collision in the crease after Senators defenseman Travis Hamonic pushed Oilers forward Zach Hyman onto his right leg.
"I just think this is rehab, but it's long," Senators coach D.J. Smith said. "It's not a six-month thing, but it's probably more like a two- to three-month thing."
Mads Sogaard started and made 34 saves for the Senators in a 4-3 overtime win against the Calgary Flames on Monday. The 22-year-old, who made three saves in relief of Forsberg on Saturday, went 1-1-0 with a 3.07 goals-against average and .889 save percentage in two NHL games last season.
Kevin Mandolese was recalled from Belleville of the American Hockey League on Sunday. No. 1 goalie Cam Talbot, who hasn't played since Jan. 25 because of a lower-body injury, is skating and is expected to return within the next two weeks.
"Obviously, you never want to see a guy go down," Sogaard said Sunday. "[Forsberg] has been so good to me in my time here, and going back to last year, too. I think me, him and [defenseman Erik Brannstrom], for all my road trip games, I'm probably out to dinner every single night with those guys, so he's really taken me under his wing and taught me a lot. I told him early on he's going to be pretty tired of me because I keep asking him a million questions. It just [stinks] for him."
The Senators (25-24-3) are seventh in the Atlantic Division, eight points behind the Pittsburgh Penguins for the second wild card into the Stanley Cup Playoffs from the Eastern Conference.
"Our details have to be bang on here," Smith said. "We can't give up odd man rushes and highlight reel stuff. We've got to make it hard for [the Flames] to score."
"For us, it's to be really committed to team defense and try to give him the best opportunity to succeed and get into the game," Senators forward Austin Watson said. "Obviously, he's super talented, he's a really good goalie. It's just a matter of finding his footing and getting comfortable in there, and I'm sure he will, quickly."
Forsberg is 11-11-2 with a 3.26 GAA, .902 save percentage and two shutouts in 28 games (25 starts) this season. A seventh-round pick (No. 188) by the Columbus Blue Jackets in the 2011 NHL Draft, he is 48-57-11 with a 3.08 GAA, .908 save percentage and three shutouts in 130 NHL games (119 starts) for the Blue Jackets, Chicago Blackhawks and Senators.
The 30-year-old signed a three-year, $8.25 million contract ($2.75 million average annual value) with Ottawa on March 21, 2022.
"You never want to see anyone get hurt," Hamonic said "There's not much to say. 'Forsy' going down ... you just try to get body position, a scramble in front of the net. Forsy is such a big part of our group on and off the ice. It's unfortunate that situation kind of happens like that. It's an unintentional accident."
Ottawa rookie defenseman Jake Sanderson is also out at least the next two weeks because of an upper-body injury. He left in the third period Saturday following a hit from Oilers defenseman Vincent Desharnais.
"We talked about how good he is, what kind of talent he is," Hamonic said. "Being a young player aside, Jake has really developed into one of our top defensemen both offensively and defensively. It's going to be a blow to our team when he's out, but it's going to be next man up until he gets back in. Everyone has to pull on the rope a little bit heavier."
NHL.com independent correspondent Callum Fraser contributed to this report