MONTREAL - The Montreal Impact announced on Tuesday that rookie forward Cameron Porter will undergo surgery on his left knee to repair an ACL tear and is expected to be out approximately nine to 12 months.

Porter, who was making his first MLS start, was stretchered off the field in the 18th minute of Saturday's 0-0 draw at the New England Revolution.

The 21-year-old from Princeton went up for the ball and his leg buckled as he came down, which left him crumpled to the turf in pain. Teammates immediately signalled for medical help.

Porter was diagnosed after meeting with Montreal doctors Tuesday afternoon and is scheduled for surgery next week.

The Impact are already without midfielder Justin Mapp, who is expected to sidelined 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.