SEBASTIAN COATES will undergo a series of tests to try to ascertain the cause of the thigh problem that forced him out of last weekend’s Capital One Cup game with Stoke City.

Coates was forced off at the half-time interval of Sunderland’s League Cup defeat, and has been complaining of pain in his thigh since.

The Uruguayan defender, who moved on a season-long loan to the Black Cats in August, is unable to take part in some training sessions unhindered, and Gus Poyet is keen to get to the bottom of the problem before it sidelines Coates for an extended spell.

Sunderland do not have an abundance of centre-half options, and while Coates is currently fourth-choice for the position behind Wes Brown, John O’Shea and Santiago Vergini, his absence with a serious injury could cause Poyet considerable problems.

“It was a precaution to take him off against Stoke, but we need to check a few more things,” said the Sunderland head coach, who has also seen Brown suffer a calf complaint. “It’s a strange one. He can do 90 per cent of whatever action you ask him to do.

“But it’s one particular action when it causes him to feel something in there. We want to see if we can do more tests and make sure that it’s right. It would be a shame if he can’t play.”

Coates’ Liverpool career was hampered by a cruciate knee ligament injury sustained around 18 months ago, but Poyet is confident the current problem is nowhere near as serious as his previous setback.

“The problem is very, very deep under the quad,” he said. “He can run up and down, and do sprinting. But when he’s in a position where he’s really fighting with a striker and over-stretches himself, or hits a long ball, that’s what causes the problem.

“He can train practically every day. I asked him one day to train and only pass it with his left foot. I asked him if he felt it, and he said, ‘No’. Then in the Stoke game, he had a few fights with the striker and while he didn’t get injured or pull up, it was there so you could feel it.”

A youthful Sunderland development side suffered a 2-0 defeat at West Ham United, with Elliot Lee scoring both of the Hammers’ goals.