Having replaced Fraser Forster in December, the goalkeeper played such a key role in saving Saints from relegation that he was voted the club's player of the season by both fans and team-mates.
McCarthy said, "Obviously I've been called up for a reason and there's now four of us here, but it's a time for me to go out there and prove what I can do.
"We're away for seven or eight days, so it's enough time to sort of show him what I can do. And at the end of the day, I want to give him a decision to make. Obviously it's an unbelievable feeling to be back here.
"I've been in two squads before but that was quite a while ago, so obviously after the World Cup and everything that has gone on it's been a positive for English football.
"It's nice to be back involved and a part of it."