Cristiano Ronaldo is facing relegation to the substitutes' bench for Manchester United's UEFA Champions League clash with Celtic after manager Sir Alex Ferguson admitted his concerns over the Portuguese forward's match fitness.

Despite netting three goals for the club so far this season, Ronaldo, 23, has struggled to rediscover his top form for United since returning from ankle surgery last month.

The injury hangover and the lingering discontent amongst United supporters over Ronaldo's open pursuit of a close season move to Real Madrid appear to have combined to diminish the player's performance levels.

Progress obvious
But while Ferguson insists that he is happy with the progress being made by Ronaldo, he admits that he is reluctant to over-work the player by playing him too often as he bids to recover full fitness.

Ferguson said: "Ronaldo is fine and he is coming on terrific now, but I spoke to Carlos Queiroz, the Portugal coach, on Saturday morning before the West Brom game to ask him what he thought.

"Carlos said that he felt that Cristiano was still finding it difficult to recover for a Wednesday game after a Saturday game.

"He had played for Portugal against Sweden on the Saturday and Carlos said he was unbelievable - his decision-making, maturity, everything was at the top of his game. But against Albania on the Wednesday, he was flat because he is still trying to find the recovery thing properly.

"So we are looking at the situation against Celtic because it is Saturday-Tuesday in terms of games.

"If we had an extra day, it would be perfect for us, but we are just looking at the situation and whether we should bring in Nani for this one. We have the option of bringing Nani in. He only played 36 minutes for Portugal last Wednesday and then fifteen minutes for us on Saturday, so he is fresh."

Ronaldo's muted goal celebration on Saturday after scoring in the 4-0 win against West Brom suggested that he was not totally happy with his lot at Old Trafford.

Fergie pleads for time
And although Ferguson refused to be drawn on the player's on-field demeanour, he admitted that the former Sporting Lisbon youngster needs time to get to back to his best.

He said: "It's just a matter of getting the games in (for Ronaldo). When players have been out for so long, and he did have his operation, there is that little response to playing all the time.

"The adrenaline can perhaps carry through the one game, but then the form dips a little bit. But he has been great and, the one thing about Ronaldo is that he can always score for you. It's almost there and he is coming along fine."

Celtic's woeful away record in the Champions League has seen them lose 17 and draw one of their 18 away games in the competition, but Ferguson insists they remain a threat - particularly in the form of Japanese midfielder Shunsuke Nakamura.

Ferguson said: "Celtic are missing the big focal point in their team with Georgios Samaras and Jan Venegoor of Hesselink injured.

"They had that two years ago and got their first goal through it. They ruffled up Rio Ferdinand and got the first goal and that set the tone for the game.

"We recognise that Nakamura is an exceptional talent at set pieces, though. He scored two free-kicks against us home and away in 2006, so I would be very surprised if we lost a goal again like that having discussed it so many times since then! There have been a few rockets given out.