Arne Engels has completed his transfer to English Championship side West Ham United.
The midfielder joined Celtic from Augsburg two years ago in a then club-record transfer worth £11 million.
Engels departs two years later for £22m, having recorded 100 Celtic appearances and winning four major honours, not to mention reaching both the knockout stages of the Champions League and Europa League.
"Naturally disappointed that he's leaving the football club," manager Martin O'Neill said on Friday. "He's done very well for us, particularly in the couple of spells that I've been here."
Celtic recently signed German midfielder Mika Baur from Paderborn in a deal that could rise to £6.5m with add-ons, but O'Neill stressed that he isn't a like-for-like Engels replacement.
"We always felt that if Arne left, we would be looking for a box-to-box midfield player with real physicality," he explained. "And that is something we would still look at."
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Engels' departure comes shortly before Celtic's crucial Champions League playoff against LASK, with the first leg of the tie coming this Wednesday in Glasgow.
