Martin O'Neill believes the loss of Alan Thompson for next week's showdown with Anderlecht is a "serious blow" to Celtic's hopes of reviving their Champions League campaign.
The in-form English midfielder sustained a calf injury in the victory over Aberdeen on Saturday and a scan last night has revealed a strain which will sideline him for at least the next fortnight.
Celtic boss O'Neill conceded: "Alan had a terrific last season for us and then he picked it up exactly where he left off when we started the new campaign. "He is a very influential player for us and there is no doubt that his injury is a serious blow for us.
"He had completely recovered from a hamstring injury which was perfectly fine by last Tuesday, and now he is sidelined with a calf problem which is a real disappointment for all of us."
Thompson was due to be watched by England assistant coach Tord Grip next week as Sven-Goran Eriksson ponders calling him up to his squad for the friendly with Denmark at Old Trafford on November 16th.
Celtic were also looking to the 26-year-old to help gain revenge on Anderlecht after inflicting defeat on them in Belgium last week. Thompson will also definitely miss the forthcoming Bank of Scotland Premier League matches against Kilmarnock and Dunfermline.