Rio Ferdinand is struggling to be fit for Manchester United's clash at Arsenal due to concussion.

The Daily Mail says the United defender must pass a series of rigorous cognitive tests by the club's medical department this week to be available for the match at the Emirates.

Ferdinand claimed on Twitter that he 'could not remember' what happened during United's 3-0 victory over Bolton at Old Trafford on Saturday.

He also admitted he had suffered concussion, which under previous FA rules meant he would automatically miss the next 10 days.