It's a genetic disorder, and chances of a full recovery are 80 percent, said Nill, who cautioned the surgery would reveal a lot more about Janmark's long-term prognosis.
"What happened with Mattias is he had a small segment, approximately 21 millimeters by 11 millimeters that became displaced and is locked in his knee," Nill said. "[The doctor] is going to take that bone segment and reattach it to the area where it came from. It's the bone and the cartilage, they both came off together.
"Healing and rehabilitation is approximately 5-6 months. Possible return in April … we're looking at maybe being available during the [Stanley Cup Playoffs]. We'll have to play that by ear."
Janmark, 23, will be on crutches for 4-6 weeks before any activity can take place, and he won't be reassessed until then.
Janmark was expected to play in a preseason game on Wednesday but left the ice shortly before the end of practice. On Thursday, he was seen at the Stars practice facility on crutches.
"We found out a month ago he was having some knee troubles," Nill said. "The doctors said there was a possibility this was starting to develop this way, and then it just happened the other day when he was skating."
Skating had no bearing on the condition, said Nill, who added it could have happened "walking into his apartment."
Janmark was a surprise roster selection out of training camp last season, and had 29 points (15 goals, 14 assists) in 73 games as a rookie. A third-round pick (No. 79) in the 2013 NHL Draft, he was expected to play a larger role early this season after injuries to centers Tyler Seguin (heel) and Cody Eakin (knee).
"It's obviously a big loss," Stars coach Lindy Ruff said. "[Janmark] was a big part of our team. He played a lot of different positions and lot different situations. I think he was a real important piece for us."
Ruff said Janmark is OK and he met with the Swede yesterday after the diagnosis.
"First and foremost, you feel bad for the kid," Ruff said. "You've got to hope he gets healthy. That is the most important thing."
The Stars' other injured players are expected to heal sooner. Seguin is supposed to start skating next week, and forwards Devin Shore (ankle) and Ales Hemsky (groin) have started skating and are expected to be ready for the regular season. Eakin and rookie center Jason Dickinson (hip) will miss the start of the season and are expected to return in November.
