The 21-year-old from Princeton went up for the ball in the 18th minute of Saturday's 0-0 draw at the New England Revolution. His leg buckled as he came down and Porter crumpled to the turf in pain. Teammates immediately signalled for medical help.
"Let's keep our fingers crossed," Impact coach Frank Klopas said after the game. "I don't want to speculate. Hopefully we can get some good news when we get back to Montreal and he's seen by the doctors."
The injury came in Porter's first MLS start.
"First and foremost, I am heart broken for @Cam39Porter. That's the ugly side of sport sometimes. Send him well wishes if possible," Impact goalie Evan Bush tweeted after recording his first shutout of the season.
To make matters worse for Montreal, defender Hassoun Camara was handed his second yellow card in the 61st minute after pulling down Juan Agudelo.
New England (0-2-1) has started consecutive seasons with three straight scoreless games — both with a 5-0 goal differential. Montreal (0-1-1) is goalless after its first two games, including a 1-0 defeat at D.C. United.
New England's Chris Tierney sent in a bending cross in the 72nd minute, but an open Lee Nguyen sailed a close-range header over the crossbar.
The grounds crew cleared snow off the field prior to the match.
The injury news has been bad for Montreal recently with midfielder Justin Mapp expected out for four months after surgery to repair a fractured and dislocated elbow.
Mapp was hurt when he fell over D.C. United goalkeeper Bill Hamid late in Montreal's season-opening loss in Washington on March 3.
With files from The Associated Press