The Sunwolves fell to their second defeat of the Super Rugby season on Saturday after collapsing early in the second half in a 37-17 loss to Australia's Rebels.

The Sunwolves went into the game at Prince Chichibu Memorial Rugby Ground looking for their first win of their third campaign in the Southern Hemisphere's premier club rugby competition, having opened the season with a 32-25 loss to the Brumbies the previous weekend.

The Sunwolves fell into an early hole when they lost key players Timothy Lafaele and Akihito Yamada to injury and conceded two tries within two minutes, but a try from William Tupou shortly before halftime helped bring the score to 10-10 going into the break.

The Melbourne-based Rebels took full control after the restart, however, running in four more tries despite Ed Quirk's late consolation score to leave the Sunwolves winless heading into next week's first away game of the season against South Africa's Sharks in Durban.

"We gave up tries too easily at the start of the second half," said Sunwolves captain Yutaka Nagare. "We had a lot of injuries among the backs so we had to reshuffle, and we had a few players playing out of their usual position. We didn't manage to communicate properly and that cost us.

"If we had been able to communicate better when the injuries happened, then it would have been better for us, but we had been practicing and preparing all week and then we were faced with a different situation."

Lafaele, Yamada and second-row forward Sam Wykes were all knocked unconscious during the game and all failed head injury assessment tests for concussions.

"Before the season started we had a lot of injuries, and last week we lost about five or six," said Sunwolves head coach Jamie Joseph.

"Injuries really affect any team. There are lots of teams in Super Rugby that have lots of injuries, but in terms of depth we are really struggling at the moment.

"All three players were knocked out. That just reflects how physical the game is. And in our case, I think a lot of it is to do with our under-preparation. Super Rugby teams have 2½ months to prepare, getting bigger and stronger. So when it comes to collisions, we are coming off second best. I think that is a cause for concern for us."

Joseph handed Japan international Michael Leitch his Sunwolves debut but the home side was dealt a heavy blow when influential center Lafaele went off in the ninth minute and the Rebels scored the opening try a minute later. Jack Maddocks touched down in the corner, although Jack Debreczeni missed the conversion.

Japan international Amanaki Lelei Mafi then scored the Rebels' second try two minutes later, and the home team's afternoon went from bad to worse when winger Yamada was stretchered off following a botched attempt to tackle hulking winger Sefanaia Naivalu.

"I had quite a lot of sympathy for the Sunwolves coaches with the number of injuries that they had, and they have had in the last couple of days," said Rebels head coach David Wessels, who coached Yamada at Western Force during the 2015 season.

Ryoto Nakamura pulled three points back for the Sunwolves with a penalty in the 26th minute, and the home side brought itself right back into contention with a try in the 35th minute. Tupou intercepted a Rebels pass right on the halfway line to run home unchallenged, and Nakamura kicked the conversion to tie the score heading into the interval.

A Debreczeni penalty and tries from Naivalu and Maddocks put the Rebels firmly back in control early in the second half, however, and when Maddocks completed his hat trick in the 52nd minute, the Sunwolves' hopes were effectively over.

"I think the set-piece mistakes in the lineout were disappointing because we trained really well," said Joseph. "We would get into a position where we really wanted to capitalize on our advantage and then we would have a small mistake in the lineout.

"What we need to do when that happens is keep it simple and get some momentum back in the game. That's what we weren't able to do quickly enough."

Quirk salvaged a measure of pride for the Sunwolves with a try in the 78th minute, but the Rebels still had time to add a penalty try before the final whistle sounded.

"I wasn't happy with the team at halftime because I thought that we moved away from our game plan, which was to be physical and direct," said Wessels. "When we did that, I thought we played well. So the big change at the start of the second half was that we tried to be more physical and more direct."