The forward is expected to need six months to recover.
"It was a meniscus situation," coach Sheldon Keefe said. "There were two different scenarios: one which they were going to remove it which would have shortened timeline to 6-8 weeks, and two, in this case they were able to repair it, which is better for him long term.
"It's a longer recovery, but if that's what's best for him, that's what's best for us."
Johnsson was injured in the first period against the Dallas Stars on Feb. 13. Keefe said the expectation was he'd be out at least eight weeks. He has 21 points (eight goals, 13 assists) in 43 games.
After providing the update on Johnsson, the Maple Leafs announced they acquired forward Denis Malgin in a trade with the Florida Panthers for forward Mason Marchment.
The Maple Leafs (31-22-8) have lost two straight games and are two points ahead of the Panthers (31-22-6) for third place in the Atlantic Division. Toronto plays the Pittsburgh Penguins at Scotiabank Arena on Thursday (7 p.m. ET; ESPN+, TVAS, TSN4, ATTSN-PT, NHL.TV).
