The stepovers and the sunny smiles have vanished but Cristiano Ronaldo is struggling with fitness, rather than restlessness at a road not taken this summer, his manager insisted yesterday.

Sir Alex Ferguson refused, point blank, to countenance any discussion on Ronaldo's present state of mind but the Manchester United manager's suggestion that he had at one stage considered resting the 23-year-old from the United side to play Celtic at Old Trafford tonight and that the winger is not yet ready to play two games a week certainly provide one interpretation of why the Portuguese could not even raise a grin when he scored against West Bromwich on Saturday.

It was Carlos Queiroz, Ferguson's assistant until he was appointed coach of the Portugal national side in July, who appears to have alerted Ferguson last Saturday to the full extent of the player's struggle to recuperate from the surgery to his right ankle which he underwent three months ago. "[Carlos] said he felt that Cristiano was still finding it difficult to recover for a Wednesday game after a Saturday game," Ferguson said. "He had played for Portugal against Sweden on the Saturday and Carlos said he was unbelievable, 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."

What wouldn't Ferguson give for Queiroz to be around at this stage in Ronaldo's United career? He considered his No 2 critical to the general contentment of United's Portuguese-speaking contingent and Ronaldo's certainly remains a worry for many. Ferguson's prickliness about suggestions that the Portuguese might have remained at United in body but not in spirit, following the talking-to he gave him in Lisbon at the height of the Real Madrid love-in, in July, certainly adds fuel to the flame. Ronaldo again cut a solemn figure when he went on to the club's TV station last night to assert, rather unconvincingly, that he is committed to United. "I think I'm an ambitious person," he said. "If I have an opportunity to be better than I am, I will try to be. I'm at the right club playing with the right players."

For his own part, Ferguson supported his argument by disclosing how, when he instructed Gary Neville – a 64th minute substitute on Saturday – to run across and ask Ronaldo how he was feeling, the Portuguese replied that it was "maybe time I come off". Five minutes later, he told Neville he felt fine. "Maybe Gary suggesting he was coming off just lifted him a bit," Ferguson said. "But that just shows you the kind of human being and what kind of athlete Cristiano is. When he needs something, he finds it."

Whatever the true state of the relationship between club and player you will not find Ferguson, who has Patrice Evra missing because of the hamstring injury picked up on Saturday, brooding much on the eve of a match like this. It sees the English and Scottish champions colliding for the first time since Shunsuke Nakamura's legendary free-kick at Celtic Park saw the Scots avenge a 3-2 Old Trafford defeat in the Champions League group stage, two years ago. The tartan invasion had started last night – 3,500 fans will descend on Manchester with tickets, hundreds more will do the best they can – and the atmosphere will soar way beyond the sedentary levels which predictable Champions League football has started to breed at Old Trafford.

Ferguson was leafing through an old St Mirren programme from his brief managerial stint there before discussing tonight's match – "all our yesterdays," he sighed, laughing at an hilariously staged picture of him on the telephone. Of course, his stint at Love Street ended in acrimony – The Saints were the only club to sack him. The way that Rangers ultimately rejected Ferguson cut him in a life-defining way, too, and there is always a sense in these cross-border counters that he has something to prove.

"I'll get my usual great reception at Celtic Park," Ferguson said, looking ahead already to the return match in two weeks. "I might go into the centre-circle this time to get my applause. It's a local derby. It's Scotland-England, but Scottish and English have the same mentality and both teams will have a go. That's in the nature and it's in Celtic's history."

The nature of the occasion – one which will see Celtic serve up some of the kind of blood and thunder uncommon in European contests – renders obsolete the club's respective European home and away records which, as one Scottish journalist put it to Gordon Strachan yesterday – leaves some people "worrying" about what actually might befall Celtic tonight. While United have gone 16 home matches undefeated in the tournament, Celtic are tiring of being told that they have never won away in five campaigns and 18 matches, with the solitary point they clinched – against Barcelona in the Nou Camp under Martin O'Neill in November 2004 – coming from a free-kick.

Though his attempt to pronounce the player's name failed spectacularly, Ferguson rightly pointed out that Celtic are missing, through injury, the "big focal point in their team" which Jan Venegoor of Hesselink provided at Old Trafford two years ago. He opened the scoring and "ruffled up" Rio Ferdinand. "They don't have that focal point so I will have to work out what they are going to do about it," Ferguson said.

You sense that there is not too much love lost between Ferguson and Strachan – "Spend time in his company? I don't know where you've been," he joshed when asked if would discuss their relationship – and there was a deft put-down for Strachan's three Scottish titles as he discussed the improvement in his own side since the last so-called "Battle of Britain". "It is not easy to win leagues down here but we have done it well," Ferguson said. And was the just verbal jousting. Let battle commence.