McDonald has hit six goals since switching from Motherwell and Caldwell said: "Scott has done a fantastic job.
"He's on fire and proved all the doubters wrong. I was delighted when we signed him.
"He is awkward and has that style centre-halves don't like in small players. He gets around your feet and causes problems."
Caldwell said: "Scott got a goal against Spartak Moscow and against Shakhtar Donetsk he put in a shift and worked hard all night.
"He'll be looking forward to Wednesday - just like the rest of us."