Sydney Roosters coach Ricky Stuart has labelled Sunday's 16-12 trial win over St George Illawarra "a costly weekend for the club" after winger Joel Monaghan sustained a season-ending knee injury.

In the final hit-out before the season opener, the Roosters dominated an entertaining clash at WIN Stadium between the premiership heavyweights in front of a crowd of 6,722.

But the loss of Monaghan who ruptured the anterior cruciate ligament of his right knee, is a bitter blow for the Roosters as they prepare to take on South Sydney in their NRL season opener at Telstra Stadium on March 12.

Monaghan, who scored nine tries in 24 appearances last year, collided with Dragons winger Wes Naiqama when charging through to contest a bomb in just the sixth minute and was stretchered off the field.

"It's very disappointing for the club, it's very disappointing for first grade," Stuart said.

"He was playing extremely well.

"He's our No.1 winger. He played 24 games for us last year, had a terrific off-season.

"It's a very costly weekend for the club."

A devastated Monaghan was too upset to speak to media after the game.

"I don't think reality has set in for him," said Stuart when asked how the speedy No.5 took the news.

Both teams rested their star five-eighths, with Ben Hornby slotting in for Dragons captain Trent Barrett, while youngster Josh Lewis played at second receiver in the absence of prize recruit Braith Anasta. Dragons prop Jason Ryles was also rested.

Returning from a hamstring strain, star centre Matt Cooper immediately made an impact, creating space down the left hand side to set up acting fullback Clint Greenshields for the opening try of the game.

The Roosters hit back seven minutes later through a converted try to winger Ryan Cross to take a 6-4 advantage.

Both teams struggled to control the ball in wet conditions, with the Roosters squandering two opportunities to go further ahead.

Centre Sam Perrett knocked on over the tryline midway through the half and then Ashley Harrison did likewise at the half hour mark.

They finally converted possession into points six minutes before the main break when Cross touched down for his second try in the left-hand corner.

The lead was then extended to 16-4 when Jamie Soward scored off a Brett Finch kick just 50 seconds before the halftime siren.

St George Illawarra hit back through a four-pointer to Naiqama 11 minutes into the second half to bridge the gap to eight points.

The Dragons winger set up a thrilling finish when he scored his second eight minutes from fulltime after finishing off a spectacular 45-metre burst from Mark Gasnier.

Dragons coach Nathan Brown was not overly concerned with the loss.

"You wake up in the morning and look at the premiership table and we are all on nought," he said.

"We're going OK, we've got some work to do."

In another knock to the Roosters Former Rabbitoh Harrison may have a case to answer after collecting Dragons forward Michael Henderson high midway through the first half.

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