Under the current terms of the deal that has taken the Manchester United midfielder to Portman Road, he will return to the Premier League champions in January.
"Talking about that is irrelevant at the moment. The deal was we'll take him to January and, if it fits, we'd go longer," the East Anglian Daily Times quotes McCarthy as saying.
"It will come down to whether Tunners is enjoying himself – which he is, he's loving it – and whether we want him, and we're certainly loving having him here.
"If the loan can be continued then so be it, but if Man United want to bring him back and play him then so be it. If that happens, then we've done a good job. We can't do owt about it until January."
Tunnicliffe, 20, has made 10 appearances so far this term.