The Australian international, 25, was signed from Motherwell in the summer of 2007 and ended up Scotland's top scorer last term with 31 goals, helping Gordon Strachan's side to their third Clydesdale Bank Premier League title in succession.

McDonald has come in for some criticism this season for failing to replicate that goalscoring form, although his plight has not been helped by the constant change of strike partners.

Jan Vennegoor of Hesselink and Georgios Samaras have picked up injuries in recent weeks leaving McDonald to play a lone striker's role against Manchester United in the Champions League last Tuesday and then partner Irish teenager Cillian Sheridan against Hibernian in the SPL.

And in Tuesday's Co-operative Insurance Cup quarter-final clash with Kilmarnock at Rugby Park, McDonald teamed up with Shaun Maloney and scored the first goal in a 3-1 win to take his tally to four for the season.

Caldwell, who is looking to seal a new deal that would see him finish his career at Celtic Park, praised his team-mate, saying: "When Scott came in people didn't really have any expectations but he showed what a good player he is.

"To raise your game again and keep that going, that is the difficult bit in football.

"He's come through a period where he has not scored as many goals but he has always been a good team-mate and he was excellent against Kilmarnock.

"He has stepped up to the plate with the bigger guys out injured.

"We have shown what a good team we are at passing the ball and Scott is very good at holding it up and he's a handful for every defence."

With skipper Stephen McManus, Chris Killen and Marc Crosas also missing through injury for the trip to Rugby Park, Strachan utilised his squad, giving the first start of the season to defender Darren O'Dea with midfielder Massimo Donati making a rare appearance.

Caldwell insists the quality contained within the Parkhead side's squad is coming to the fore.

He said: "It was a tough cup tie and we had to be right mentally.

"I thought the attitude was first class and the performance was very good.

"We have the squad to rotate and anyone who comes in seems to do the job as good, if not better, than the next person.

"We are playing good attacking football and scoring lots of goals and it's good to be a part of it."

Strachan also praised his players' adaptability.

"They should be happy with they way they played," he said. "Their movement and bravery on the ball was terrific and it was good to see good football being played."

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Strachan's injury problems deepened tonight when Dutch defender Glenn Loovens revealed he would miss the next two or three weeks with a thigh injury.

Although only a fringe player at present, Loovens' loss robs Strachan of vital cover going into Sunday's SPL trip to Hearts and Wednesday's Champions League return against Manchester United.

"The club told me a few weeks so hopefully it goes a bit quicker than that," the player told STV.

"It's never nice to be injured with those nice games coming up but that's football."

Loovens has urged his team-mates to go for broke against United, with a win vital to the club's slim qualification hopes.

He said: "That's our last chance so we really have to go for it. It's going to be a different game with our supporters behind us."