After a number of loan spells taking him from Yeovil to Leeds, McCarthy has impressed in the Royals' first-team despite only making 11 top-flight appearances this term after undergoing shoulder surgery.
The 23-year-old goalkeeper - who is being linked with a summer switch to Liverpool, Newcastle and Fulham - was outstanding again in Saturday's 4-2 win at the Cottagers and Karacan is surprised he had not yet had a call-up to the full England squad.
He said: "He's amazing - unbelievable.
"It's surprising for myself when you see the likes of Jack Butland getting called up ahead of him.
"I think he showed it again with three unbelievable saves. You can see the top clubs knocking on the door.
"But he is playing here and proved himself to be an unbelievable goalkeeper in the Premier League.
"It would be a coup if we could keep him, but he loves it here, is playing here and that's all he ever wanted to do.
"You never know. He will get a lot of games next year, but there will be clubs knocking on the door and definitely a future number one."
Asked how much he is worth, Karacan added: "Around 15 to £20million? You see how much young English players go for - there are some crazy sums out there.
"I don't see why he shouldn't be in that bracket. It would definitely take a lot of money to get him away from Reading - that's how our club works."