New York City FC coach Patrick Vieira said the "electric" atmosphere at Toronto FC's BMO Field had an affect on the referee as his side fell to a 2-0 defeat in Sunday's first leg.
Late goals from Jozy Altidore and Tosaint Ricketts gave Toronto a 2-0 advantage ahead of next week's return leg at Yankee Stadium.
NYCFC were penalized with five yellow cards to Toronto FC's one, and Vieira suggested referee Silviu Petrescu may have been influenced by the crowd.
"They put a lot of pressure on us - it wasn't an easy game at all," Vieira said. "The atmosphere was electric, their fans were incredible throughout the 95 minutes.
"The referee was a little bit harsh on us and I think the occasion had an effect on some of his decisions, especially in the moments we were in difficult moments.
"They played very well and to concede the two goals late on is very frustrating."
Vieira chose to start 23-year-old Mikey Lopez over Frank Lampard in midfield, though the former Premier League star did come on after Lopez was booked -- and was perhaps fortunate to escape a red card -- for a second-half challenge.
Lampard said he was fit to play 90 minutes, but Vieira stood by his lineup choices.
"We had a game plan and I wanted to be more solid centrally and that's why I chose Mikey to play in this game," Vieira said. "We knew it would be a difficult game, we've played them already here. We know the quality of their players and the physicality of the game. I thought we managed it well until the first goal.
"I thought we managed their forwards quite well until then but the game is 95 minutes and we gave them a chance and they punished us right away."
Andrea Pirlo was unavailable with a calf strain, and Vieira said "we didn't want to take a risk" with the midfielder, who is set for X-rays over the next week.
Vieira was also questioned on his decision to replace star striker David Villa with Khiry Shelton in the 78th minute.
"I wanted to bring fresh legs on up front and some height, we struggled to hold up the ball. I wanted some more physical presence up there," Vieira said.
"It's difficult to put a finger on anything specific -- David didn't touch the ball many times. His movement could be better to get the service. I wanted a bit more physicality up front."
Ricketts' stoppage-time goal doubled the task for NYCFC in the second leg, but Vieira remained confident.
"It makes it really difficult for us -- but I think I have enough experience in the game to know that things can change," he said.
"I look in the dressing room and we know anything can happen. We have to believe, we have to give ourselves a chance and go for it."