Albion tried to keep him but he opted for a move to Scotland.
"Was I close to staying? We talked at the time but we couldn't find a solution and there were some Premier League clubs, and also Celtic, who came in for me," he told the Birmingham Mail.
"I'm a footballer, I like a challenge and went to Celtic. But I never regretted it.
"Celtic were a big club and I played well up there, though I'm glad to come to West Brom.
"It's good to be back. I had a good time here, with some good memories."