Neville is expected to play for United when they face Celtic in a crucial Champions League match at Parkhead on Tuesday, and again at the weekend when they face Chelsea in a Premiership showdown.

However Ferguson is openly concerned about the long-term prospects for a player he considers crucial to his side.

The 31-year-old first reported the injury when he returned from World Cup duty with England, since when he has struggled to play a consistent run of games for the Old Trafford side.

Fluid which had been developing in his right leg has now started to appear in his left, prompting Neville's half-time substitution at Blackburn a fortnight ago.

He then missed England's friendly draw in Holland last week before returning to the United starting line-up for Saturday's win at Sheffield United.

However, while Ferguson claimed his captain suffered no reaction to his outing at Bramall Lane, on the eve of the match against Celtic, he admitted concern over Neville's long-term health remains.

And, if the injury returns, the United boss has confirmed action will have to be taken.

"Gary came through Saturday's game okay and he trained this morning but obviously, the injury has been a concern," said Ferguson.

"We are holding our breath hoping it doesn't happen again because if it does we will have to take a long-term view and do something about it."

Although Neville could carry on playing, Ferguson is not prepared to take the risk of the full-back breaking down completely, which could lead to a lengthy spell on the sidelines at a time when United are chasing glory on what the Scot hopes will be many fronts.