Forrest enjoyed great success in Rodgers' three years at Parkhead.
"When I got to Celtic, James had only six months left on his contract and the feeling was he'd leave," Rodgers told the Daily Record.
"He'd been inconsistent for a couple of years, in terms of his form and his training between games.
"He didn't have the consistency but always had the quality. He's a remarkable player and I've always watched him from the outside.
"It was just about trying to maximise that and bring more efficiency to his game.
"James had the talent but the season before I arrived he'd only scored two goals. The year before that it was four.
"But we got him right physically then helped him mentally.
"He trained every day and enjoyed the work we did with him.
"You could see him developing and it was then about pushing him to be the best he could be.
"He got 11 goals and nine assists in the league alone last season. So in terms of efficiency, he's played consistently now at a high level for three years."