OXFORD United’s defensive resources have taken another hit after Rob Atkinson broke down in training.

It was the second time the centre back had suffered a recurrence of the groin injury he originally picked up against Bristol Rovers in the EFL Trophy earlier this month.

Both times it was thought to be a minor issue, but boss Karl Robinson admitted they would now have to investigate it more thoroughly.

He said: “That’s the third time that’s happened.

“There will have to be more of an extensive look at that to see what is right and wrong.

“It’s only just happened so it’s difficult to comment on, but we’ll have a meeting to go through it.”

Losing the 22-year-old is a blow to a unit which has been plagued by fitness issues since the start of the season.

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Atkinson began the campaign looking set to step up and become part of the first-choice centre back pairing, alongside Elliott Moore.

But Sam Long has been the one to step in during recent games, with the experienced John Mousinho another member of the squad nursing an injury.

The club captain was drafted in on the bench at Charlton Athletic on Tuesday night, but Robinson had felt it would be another week before Mousinho was ready to feature.

With Sean Clare and Josh Ruffels also missing games through injury, the unsettled back back four have conceded an average of two goals per game in Sky Bet League One.

Elsewhere, United are hoping a re-think on Sam Winnall’s training will get him up to speed.

The striker did not sign until after pre-season and has suffered several niggles while attempting to get up to speed.

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Instead, the U’s have opted to take a step back and aim to reintroduce him in the Emirates FA Cup and Papa John’s Trophy games in the week after next.

Robinson said: “Sam feels like he’s getting into a better place.

“He’s not done any sort of running work, so he’s on a pre-season plan now.

“He’s hoping to be involved in the two cup games, one from the bench and one from the start.

“We go 60 minutes and 30 in pre-season, so we’re going to try to do that for him in those two games.

“That will be him ready and raring to go.”

He added: “We were travelling away to different games and you wanted to make sure you did something with him, then is that too much or too rushed?

“I think if we have a clear plan it will hopefully negate the little niggles he’s picked up.”