The 21-year old centre-back had a successful loan spell with Huddersfield Town last season, helping the Terriers into the Championship, but Karacan is backing him to make a positive impact for the Royals now they are in the Premier League.
"I love the geezer. He's one of my pals and it was sad to see him go to Huddersfield last year," he said.
"He was huge in getting them promoted and he's a colossal guy on the pitch. Sean has only just turned 21 and he will definitely be a player to watch out for.
"It's great to have him back and I think the gaffer knows that he can have a big future here."
Karacan added: "I'm sure he will have a few clubs in for him - he's a brilliant young defender.
"Whether he needs to go out on loan again, I guess time will tell, but I'm sure Reading fans will see a lot more of him in the future."