Republic of Ireland striker Long completed a brace with a late winner, but the victory was only possible after goalkeeper Federici made a string of superb saves to thwart the Rams.
"It was a great performance from the lads," Karacan told Reading's official website. "At one point we were under a lot of pressure and Feds made some ridiculous saves.
"When I looked at the highlights, he doesn't just save them - he pushes them out of the dangers area too.
"I'm delighted for Shane, he's been so close in recent weeks and it show how important he is for us."
Long earlier opened the scoring from the penalty spot, and Karacan added: "It was a great penalty, when he gets it you know he's going to score it."
Source: PA
Source: PA