Toronto FC striker Jozy Altidore suffered a hamstring injury in his team's 4-3 defeat to the Vancouver Whitecaps on Saturday night.
Vancouver was already ahead 2-0 when Altidore had a penalty attempt saved and immediately exited the game. Altidore will now be in a race to be fit before the United States open play in the Copa America on June 3. Final squads for the tournament must be submitted by May 20.
"I don't know specifically how he is," TFC coach Greg Vanney said in his postmatch news conference. "He said as he was approaching the penalty, on the take-off step, he felt it a little bit... [His hamstring] is the indication that I have. We'll see."
Altidore injured his hamstring in the first game of the 2014 World Cup, then suffered a similar injury before the 2015 Gold Cup and also missed the first two games of the MLS season.
"You don't wish injuries on anyone, especially teammates," Michael Bradley said. "We wait to see what comes of this and go from there."
After Kekuta Manneh and Christian Bolanos scored inside the first 18 minutes, Sebastian Giovinco then scored twice to pull Toronto even, moving into sole possession of the franchise record for career MLS goals with his 29th and 30th strikes.
But four minutes after Giovinco's tying goal in the 66th minute, Manneh restored the Vancouver lead, running past former Whitecaps full-back Steven Beitashour before beating goalkeeper Clint Irwin. Morales added an insurance marker two minutes later.
Vancouver full-back Fraser Aird was sent off in the 75th minute for a second yellow card, while midfielder Jonathan Osorio was hurt on the play and had to be helped off. Drew Moor's header off Giovinco's corner in the 80th minute cut the deficit to 4-3, but Toronto could not find an equalizer.
Lee Nguyen scored on a right-footed blast in the 22nd minute as the New England Revolution beat the Chicago Fire 2-0.
Nguyen's goal came after he played his corner kick short and slipped unmarked into the left side of the area. Scott Caldwell fed it back to Nguyen, who one-timed it into the far corner of the goal.
Femi Hollinger-Janzen made it 2-0 with a header for the Revolution in the 84th minute.
Kei Kamara made his New England debut two days after being acquired in a trade from Columbus Crew SC. Kamara scored 22 goals for Columbus last season, tied for the most in MLS, but had a falling out with the club after an on-field dispute with Federico Higuain over who would take a penalty last Saturday.
Mauro Diaz and Michael Barrios scored and FC Dallas beat the Seattle Sounders 2-0 to end a three-game losing streak.
Diaz buried a penalty into the top-left corner of the net in the third minute after Walker Zimmerman drew a foul in the box against Seattle's Chad Marshall.
Barrios dropped the ball to Mauro Rosales, who replaced Diaz in the 79th minute, then darted behind the defense and slotted a feed from Rosales through the legs of Seattle defender Brad Evans and past goalkeeper Stefan Frei in the 80th.
Chris Seitz had three saves for his second clean sheet of the season for FC Dallas, which is unbeaten in seven home matches this year. The Sounders lost for just the second time in seven games after starting the season with three defeats.
The Montreal Impact stayed first in the East standings after a 1-1 draw with the Philadelphia Union.
In a battle between the conference's surprise top two teams, Didier Drogba needed only three minutes before opening the scoring for Montreal.
Marco Donadel's corner was headed by C.J. Sapong straight to Drogba at the edge of the penalty area. He took one touch and blasted a shot just under the bar for his fourth goal in seven games this season.
The Union answered in the 24th minute as Laurent Ciman's slip allowed Chris Pontius to cross from the left side to an unmarked Sebastien Le Toux to feed Sapong in front for a tap-in.
Despite their place atop the table, the Impact have drawn four straight and are winless in their past five. The Union had their third straight draw to run their unbeaten streak to four.
Kevin Doyle scored with a header to help the MLS-leading Colorado Rapids draw 1-1 with Columbus Crew SC and extend their unbeaten streak to seven games.
Doyle's header, from the top of the six-yard box, found the bottom-left corner of the net following a corner kick from Shkelzen Gashi in the 55th minute to make it 1-1.
Just seconds after Cedrick Mabwati had a shot from the top of the box deflected, he put away a pass from Harrison Afful to give Columbus the lead in the 23rd minute.
The Houston Dynamo's home game against Real Salt Lake was postponed until Sunday at 2 p.m. ET afternoon after major thunderstorms hit the area.