Romero suffered the injury on Saturday in Colorado, during the first half of the game against the Rapids.
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He is expected to be out approximately 6-9 months.
After a breakout 2014 season when he was named team MVP, Romero had a steady 2015 season with the Impact. In 27 regular-season games, he scored four goals and collected four assists for an Impact team on the verge of clinching a Audi 2015 MLS Cup Playoffs berth. He also scored two goals in three appearances in the knockout stage during Montreal's run to the CONCACAF Champions League final.
The Impact can clinch their second trip to the playoffs with a win over the New England Revolution on Saturday (7:30 pm ET, RDS, TSN2 in Canada, MLS LIVE in US).