Toronto Raptors head coach Nick Nurse is optimistic that the NBA's defending champions have the mentality to bounce back despite the Boston Celtics taking a 3-2 playoffs lead with a 111-89 victory on Monday.

The Celtics won the first two matches in the Eastern Conference semi-finals series before the Raptors battled back to level it up at 2-2 with successive wins of their own.

But now the 2019 champions are facing the prospect of a do-or-die Game 6 following Monday's loss, knowing defeat next time out will see their defence crumble.

Nurse feels positive about their chances, however, adamant they have shown their mettle when it really mattered in the past.

"We're really good at bouncing back," Nurse said.

"It's strange to me that we have these kinds of games, it really is. But historically we have [bounced back], and then historically we really usually bounce back in a big way."

Frustrations boiled over in the Raptors' ranks during the match, with Serge Ibaka reprimanding Kyle Lowry for a personal foul on Jayson Tatum before Fred VanVleet attempted to calm the former.

VanVleet insisted the situation was just accentuated by the fact there were no fans there, however.

"Those frustrations happen all the time," he said. "You guys don't see a lot of them, but that was one that was obviously visible and you could probably hear a little bit there if you were in the arena. Just some frustrations with the game.

"Kyle had his moment with the ref. Serge had his moment with Kyle, but we moved on, we moved past it. It happens. We're brothers.

"It wasn't as bad as it looked, I can tell you that much. That's an everyday thing on our team. Guys just being honest. We have a lot of passionate guys. It's just part of the process."

On the flip side, Jaylen Brown – whose 27 points led the way for Celtics – is not getting carried away, reminding his team-mates they still have a job to do in Game 6.

"It's the playoffs, we've got to come out ready to fight every single night. If not, that's how you lose. So, our guys came out ready to fight tonight," he said.

"But [Monday's result] doesn't say anything. We've got to get ready to play [Wednesday]. The job's not finished yet. We've still got a lot of work that needs to be done."

Coach Brad Stevens added: "They've got a lot of people to go to, and we're going to have to be good again."