The 6ft 7ins attacker was made available for transfer at the end of last season having played just eight times for the club in almost two years.
McCarthy said in the Express & Star: "We made a lot of substitutions and Stefan was among the pick of them - he was excellent.
"I bought him to do that and if he continues to do that then he will be in my squad."
However, German club Eintracht Frankfurt are interested in the 28 year old Austrian striker, with sporting director Bruno Hubner commenting: "A guy almost two metres tall can win you the odd game through his dynamism and heading ability.
"I can't deny it, he's an interesting player."