The South American has been the star performer for The Black Cats this season, but McCarthy fears that promotion is the only way that Arca will stay.
The 23-year-old, who arrived at the club in July 2000 from Argentinos Juniors, has just over a year left on his current contract.
"Julio Arca has a year left to run on his contract come the end of this season," said McCarthy.
"If we should be good enough and fortunate enough to be in the Premier League next season, it won't be an issue.
"If we're not then it could be an issue - because with one year to run on his contract, you are then having to sit down and discuss whether he wants to stay.
"With just one year to run, if he's not going to sign another contract that does become an issue, just like with any other player."
