The Belgian international, signed from Anderlecht for £2.5million, can also play left side of midfield or at centre-back.
But the Wolves boss has no doubts where he will play his new signing.
McCarthy told the Wolverhampton Express and Star: "Jelle is a left-back. He can also play wide left or as a left centre-back, but predominantly he is left-back.
"The day I saw him play, he was terrific on the left side of midfield.
"He played like an English league player - headed it, tackled and competed. There were no airs or graces about him.
"He's got to be a threat in the air. He's 6ft 3ins and he's got goals in him.
"Physical size and strength is important in this league."