"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."
Having destroyed Dundee United, McDonald's next assignment is to shoot down European champions AC Milan.
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."
