The Greek has joined West Bromwich Albion on a two-year deal to return to the Barclays Premier League for the first time since 2008. In his first spell he scored just 12 goals in 65 games for Manchester City after a £6m move from Heerenveen in 2006 before being offloaded to Celtic two years later.
"I'm more mature now and know football better than when I was younger," he said. "I had seven beautiful seasons at Celtic and I'll keep all those good things.
"I know football better now. It was an easy decision for me to come here. I had a really nice chat with the gaffer [Alan Irvine]; he's Scottish so we spoke our language."