The defender suffered a "cruciate injury" in her knee during training last week in Cyprus, according to a Canadian Soccer Association spokesman. She has returned home to Wisconsin for further testing and treatment.
The injury has left Sesselmann's season with the NWSL's Houston Dash in doubt, and is a big blow to the Canadian squad on the eve of the Cyprus Cup tournament.
"That's my only natural left foot on the whole team, believe it or not. We do lose a lot with Lauren," coach John Herdman said. "She's versatile and she genuinely does give you that natural left foot that plays quite instinctively."
The 30-year-old Sesselmann has 39 caps for Canada and played on the Canadian squad that won bronze at the 2012 London Olympics, starting all six matches. The aggressive five-foot-eight defender is the second recent big blow to a Canadian women's team that recently lost Kara Lang to a third devastating knee injury.
"Any time we lose a player it affects us emotionally," said goalkeeper Karina LeBlanc. "I think what Lauren brought is that aggressiveness, that toughness and she had leadership in her own way.
"For us now, it's another person that we play for and it's tough that she went down because she was playing phenomenal soccer. You never want to lose a teammate but when someone's doing so well and starting to really find their own way it's bitter. But the great thing is that she's going to have time to come back before the World Cup and she's going to make one of those great comeback stories."
Canada is hosting the 2015 women's World Cup.
Sesselmann posted on her Twitter feed on Sunday: "A huge thank you to my #CanWNT, family, fans and friends for the well wishes!! I promise I will work my butt off to get back on field soon!!"
Lang, meanwhile, was attempting a comeback after two torn anterior cruciate ligaments forced her to retire at the age of 24. The 27-year-old from Oakville, Ont., launched her comeback last March and had just been cleared to train with the national women's team when she tore "everything you can in a knee" last month.
Canada plays Finland in Lanarca on Wednesday to open the eight-country Cyprus Cup. The Canadians then play Italy in the same city on Friday and England in Nicosia on Monday.
Herdman said while the team loses a little bit of balance in Sesselmann's absence, it will mean more time for younger players such as Kadeisha Buchanan, an 18-year-old from Mississauga, Ont., who has shone for the senior side since she earned her first call-up last summer.
"This is what happens in tournaments: you have to adapt and overcome," Herdman said.
The Canadians are aiming to reclaim the Cyprus Cup, a tournament they have won three times but haven't won since 2011. England beat Canada 1-0 in the last year's final, and France won in 2012, with a 2-0 victory over Canada in the final.
Herdman expects a tough challenge from Finland, which is unbeaten in its last five games.
"They're a good team," Herdman said. "I watched them through the Euros and they had some good results there. . . so they're coming into this confident and ready to play their style and they're a very aggressive, forward thinking, counter-attacking, and direct team.
"That's not being disrespectful to them. They are so organized in their strategy that we really have to be on our toes. If we make any little mistakes around the back, this team punishes you."
The coach added that getting off to a strong start is crucial.
"We're going to have a tough game against Finland, an even tougher game against Italy and then an even tougher game against England so there isn't really a team in this group where you're looking at giving your top players a lot of rest," Herdman said. "They're going to have to play."