Leclerc won the Monaco GP two weeks ago and his Ferrari team arrived in Canada as favourites for victory in Montreal given the nature of the circuit, which favours the characteristics of its car.
But both Leclerc and team-mate Carlos Sainz were eliminated in Q2 at Circuit Gilles Villeneuve, with the Monegasque finishing 11th, one place ahead of the Spaniard.
Leclerc admitted Ferrari was "nowhere" with its pace, saying he felt something was wrong with his car during final practice.
"Well, we are just not fast enough and unfortunately, that's it," said Leclerc. "I mean, in FP3 we were nowhere on the dry, in qualifying we were nowhere on the dry as well.
"I don't have any explanations for now. In FP3 already we felt that something was wrong, we couldn't see what was wrong and that was exactly the same in qualifying where it definitely felt like something was wrong but nothing we could see was wrong."
He added: "The grip was just extremely poor in the first sector especially, and then once you slide in the first sector it's a snowball effect and you never really get the performance out of the car."
Leclerc conceded the result came as a shock given Ferrari's progress in recent races.
"Very surprising," he added. "I did not expect that and it's obviously disappointing but we've got a race tomorrow. I believe that in the race the issues that we have had in qualifying will be a bit different."
Sainz was also baffled as to why Ferrari's performance had disappeared, but the Spaniard reckoned it was a combination of factors contributing to its struggles.
"I think right now, I can just tell you we are lacking grip and our ride doesn't look as good as it did in Monaco," Sainz said. "For these two reasons, lack of grip, warm-up, ride, everything around Canada seems trickier than Monaco.
"We are a bit surprised, everyone knows, because since FP3 really we saw we were slow and this weekend was going to be a tough one, and you never expect to go from fighting for a win and pole position to being out in Q2, but this is Formula 1.
"I've seen worse things happen and we will go back and analyse why we're struggling around here."
Additional reporting by Filip Cleeren and Jonathan Noble
