Argentine Martin Bravo netted a first-half hat-trick with Efrain Velarde adding another goal, all before half-time, as Pumas recorded their first win of the campaign.
The loss caused Toronto to drop to third place on the group table and forward Peri Marosevic was unable to explain the heavy defeat.
"It seems like we came out kind of flat - and it showed," Marosevic said. "It seems like none of us had it in us tonight. We just have to do better.
"Obviously, we have to forget about this game," Marosevic added. "We have to learn from the mistakes that we made, the goals that we gave up. We have to look at it and learn from them really quickly."