Sir Alex Ferguson is considering dropping Cristiano Ronaldo for Manchester United's Champions League clash with Celtic tonight because he fears the winger is struggling to play two games a week.

Ronaldo underwent an ankle returning operation in the summer and has played nine games since returning to action five weeks ago.

He played all of Portugal's World Cup qualifiers against Sweden and Albania and followed that up with another 90 minutes against West Brom on Saturday.

Ferguson, after talks with former assistant and current Portugal coach Carlos Queiroz, may now rest Ronaldo tonight.

"Carlos feels Cristiano is still finding it hard to recover from a Saturday game to a Wednesday," said Ferguson. "I need to look at this game and the fact it is on a Tuesday and assess whether to bring in Nani."

Even without Ronaldo, United still have plenty of options up front with Ferguson facing a selection dilemma as Carlos Tevez hopes to split Wayne Rooney's partnership with Dimitar Berbatov. The boss said: "It is very difficult trying to gauge whether I can play the three of them together. I have been thinking about it for a couple of weeks. It is possible, with one of them playing in behind."

Definitely out for United is Patrice Evra, who injured his hamstring against West Brom and will not be risked, so John O'Shea will fill the left-back role at Old Trafford.

He will play alongside Nemanja Vidic, who is urging his team-mates not to get distracted by the occasion tonight. The Serb played in both Champions League matches against Celtic in 2006 and said those games provided him with the best atmosphere he has known.

It promises to be the same again this evening as Celtic look to improve their own diabolical away record in the competition where they have lost 17 of their 18 games on the road.

"The champion team of Scotland will be desperate to beat the champions of England," he said.

"The atmospheres I experienced two years ago were the best I have known in my career and there will be more tension in the ground. But it is not good for the players to be too passionate. We need to be focused.

"If we put too much aggression into it or pay too much attention to the crowd, we could find it difficult. We need to stay calm and concentrate."

* Live on ITV1, 7.45pm