BLACKBURN boss Mark Hughes is expecting to be without Jason Roberts for a minimum of three weeks after a scan revealed the striker tore a thigh muscle during Wednesday night's 3-0 Carling Cup victory over Bir-mingham City.

Roberts collapsed to the ground in agony when he damaged his leg in the act of shooting early in the second half against the Blues, and a scan result yesterday confirmed the former Wigan man has suffered a grade one tear, which means he is now set to spend a number of weeks on the sidelines.

"We will have to wait for the muscle to settle down and any bleeding to run its course," said Hughes.

"But I think Jason will be out for a number of weeks. Whenever anybody pulls up sharply like he did, it's usually a minimum of two to three weeks."

After losing his place in the side to Roque Santa Cruz, Roberts had waited patiently for a chance to impress Hughes, but then when it came on Wednesday he lasted just 54 minutes before his dreaded injury curse struck again.

The 29-year-old, who cost Rovers £2.5 million from Wigan 14 months ago, also spent most of last season on the treatment table after he was struck down by groin and metatarasal injuries respectively, so this latest setback is the last thing he needed, at a time when he was looking to force his way back into Hughes' plans.

The Rovers chief said: "Last season Jason had a run of games, then he had a muscle injury and went out of the side.

"He then came back and got his metatarsal injury and that was a long one for him, and it took him a while to get.

"But once he did get back, he scored a number of goals at the back end of last season and showed some good qualities.

"This year, he's had a number of games and on Wednesday night he was looking forward to making his mark and, to be fair, I thought he did. I thought he played well before his injury.

"So it's been frustrating for him, unfortunately."

In the meantime, Hughes said his decision to experiment with a 4-2-3-1 formation against Birmingham has given him food for thought for the future.

The Rovers boss normally sticks religiously to a 4-4-2 formation, but he mixed things up on Wednesday night and after seeing it work well, he said he won't be afraid to use it again in the Premiership.

"It's something I looked at in pre-season and I actually tried it against Everton last year, when I put David Dunn in there, but unfortunately I didn't get the opportunity to see it properly because Dunny came off after about 10 minutes and we had to change it again," added Hughes.

"But I just feel it's something that, on occasions, we will be able to use.

"It gives us a little bit more variation against opponents on occasions, and we may well be able to use it."