Bayer Leverkusen chief Simon Rolfes has paid tribute to Moussa Diaby after he completed his move to Aston Villa.
Diaby makes the switch for a fee of 60m.
"Moussa Diaby developed into a fantastic player during his time at Bayer 04," said Bayer's sporting director Rolfes.
"The level of professionalism and determination with which he went about his job every day was truly exceptional.
"With his special style of play and enormous speed, Moussa has left his mark on our team in recent years. He is a person who has his heart in the right place."
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