CELTIC were dealt a severe Old Firm blow today when top scorer Scott McDonald was confirmed to have torn a muscle in his groin, The damage to the little Aussie hitman's adductor muscle now looks likely to rule him out of Sunday's meeting with Rangers at Parkhead.

And Jan Vennegoor of Hesselink was rated only 50-50 to make it after having stiches inserted in the deep leg gash he suffered during the weekend win over Falkirk.

McDonald is causing most concern. The man who hit the net 31 times last season reported the injury late last week after returning from playing for Australia against South Africa in London.

He had to sit out Saturday's win. But manager Gordon Strachan hoped that, after a few day's rest, the diagnosis might show the injury was only a slight strain.

However, tests have now shown the muscle is torn. And, depending on the severity of the damage, that could sideline McDonald for four to six weeks.

If that is the case, he would also be out for the opening round of Champions League games which take place on September 16/17.

Even with the success of Shaun Maloney and Georgios Samaras for the final hour of the game against Falkirk, it would be a massive blow to Strachan if he had to plan for this first Old Firm game without the two men who combined to score 51 goals last season.

But better news is that defender Gary Caldwell - who has been out of action since damaging a thigh muscle against Dundee United 10 days ago - is well on the road recovery and looks like making it in time to line up against Rangers.

Coach Neil Lennon delivered the latest update on the condition of Celtic's walking wounded. He said: "Scott has got a wee groin tear so he could be a doubt.

"Jan has got five stitches in an ankle injury so it could be 50-50 for the weekend. We will just have to see how he is.

"Gary had a thigh problem. But with the rest over the weekend, he should be fine."

If Caldwell suffers any reaction in training this week, Glenn Loovens is set to team up once against with Stephen McManus at the heart of the defence.

But it is up front where most concern lies, and Lennon accepts the loss of McDonald and Vennegoor for such an important game would be a massive disappointment.

The £2.5m signing of Maloney from Aston Villa could not have been better timed, and Lennon is confident his former team-mate can pick up where he left off in his sensational debut at the weekend.

Maloney set up two goals for Samaras, even though they were linking up for the first time, and he said: "I think Shaun can play with most players because he is really intelligent.

"It is the typical big man/little man partnership, and Sami responded very well.

"Shaun has played with a lot of good players over the years, and he can play anywhere across the front line."