Sir Alex Ferguson has revealed Manchester United skipper Gary Neville may require surgery to cure his troublesome calf problem.
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 outfit.
Worryingly, the fluid which had only been developing in his right leg has now started to appear in his left, prompting his half-time substitution at Blackburn a fortnight ago.
Neville 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 United's eagerly-awaited Champions League showdown with 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.
'If it keeps going on and on and on, we might get to the new year having lost him for a long period,' he said.
'Had it just been one calf, we would be saying to ourselves `give it time'. But because he is getting it in the left one as well, it suggests it is something different.'
Although both Wes Brown and John O'Shea are capable of filling the right-back berth, Ferguson is understandably keen to have one of the mainstays of his squad available and is keeping his fingers crossed the problem will rectify itself.
'Gary is our captain,' said the Scot.
'He has the experience and we want him in our team all the time.'
Given Neville's present condition and Sunday's crunch Premiership encounter with major title rivals Chelsea at Old Trafford, it would be no surprise if the defender was one of the `maximum two changes' Ferguson has threatened to make at Celtic Park.
The obvious alternatives are to bring in Gabriel Heinze for Patrice Evra in the other full-back slot or to hand Scotland international Darren Fletcher a start against the club he supported as a boy, either at the expense of Paul Scholes or Ryan Giggs.
Ferguson is wary of making too many changes having left Neville, Giggs and Louis Saha behind in Manchester and Scholes on the bench for United's last Champions League outing against FC Copenhagen, who went on to record a shock win.
'I have not made my mind up about who should be left out,' said Ferguson.
'We took a risk in Copenhagen, so we are well aware of the dangers of taking too many chances.
'There is a temptation to play my strongest side because we still have another five days before the Chelsea game.'
While United are hefty favourites to confirm the perceived superiority of the English Premier League over its Scottish counterpart, Ferguson is knowledgeable enough of life at Celtic too .
The Hoops have lost just one of their previous 11 home Champions League ties, against the might of Barcelona in November 2004.
And, with Gordon Strachan's men requiring a positive result to keep themselves in contention for a top two spot, Ferguson knows collecting the point required to secure United's passage into the last 16 cannot be taken for granted.
'Celtic Park is without question one of the best atmospheres anyone could experience and their players can be inspired by it,' he said.
'If the game is going Celtic's way, you have your work cut out to stem the tide of emotion and enthusiasm that can be generated by that support.
'Celtic's record at home is one of the best in Europe, so we accept we are in for a tough game.'