Salo was hurt early in Sunday's 4-3 win over Dallas, which was also the Canucks' first game without top goal-scorer Daniel Sedin, who is out four to six weeks with a broken foot.
"Our docs and therapist suspect it's an MCL and at first view they are thinking it's anywhere from three to five weeks," head coach Alain Vigneault said on Wednesday.
When healthy, Salo -- who has missed 223 games over previous seasons -- is the Canucks top all-around defender, logging more than 21 minutes of ice time while playing on the top power-play and penalty-kill units.
However, the 35-year-old Finn has missed 79 games over the last four seasons in Vancouver, including 22 with four different injuries last year. The Canucks were just 9-10-3 without Salo, and 36-17-7 with him last season.
Vancouver called former first-round draft pick Michael Grabner up from Manitoba of the American Hockey League on Wednesday to replace Sedin on the No.1 line. But Vigneault said a salary cap "situation will prevent them from recalling any extra defenseman," meaning journeyman Aaron Rome will replace Salo after watching the first five games from the press box.
Veteran defender Mathieu Schneider is also nearing a return from summer shoulder surgery. But because the team placed him on long-term injury in order to get salary cap relief at the start of the season, he isn't eligible to play for the first 10 games or 24 days of the season.
Vancouver is also missing winger Jannik Hansen, who broke three fingers in a fight in the final preseason game, and top-six forward Pavol Demitra, who remains out indefinitely after recent setbacks in his return from offseason shoulder surgery.
The Canucks are counting on Grabner, a speedy 22-year-old Austrian selected 14th overall in the 2006 NHL draft, to have make an immediate impact on the top line. He scored 30 goals in the AHL last year, but was sent back down after a disappointing training camp and preseason. Grabner scored four goals and added two assists in his first five AHL games to earn another call up.
"With two top-six forwards out we need someone to hopefully step into that role," Vigneault said. "Michael is an offensive player with great speed and pretty good touch around the net."