The 20-year-old has impressed during his time with the Tractor Boys with the terms of his current agreement due to see him return to Old Trafford at the end of the year.
Ipswich have been beaten just once in the seven games in which Tunnicliffe has started, leading to speculation that the Championship club could look to keep him for the remainder of the season.
But McCarthy would not be drawn on the youngster's future at present, although he insists that they have enjoyed working together so far.
McCarthy told the Ipswich Star: "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.
"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 anything about it until January."