Mackay is delighted about signing the Frenchman, hailing his contribution thus far.
"Kevin is gold as far as I'm concerned," said Mackay, who will take his side to Aston Villa on Saturday bidding to cement their position in the Premier League mid-table.
"He is the kind of player everybody wants in their side.
"He's a full-back who is adventurous, but who at the same time actually enjoys defending.
"Despite his age, he was a leader at Rennes and he's brought that level of maturity and composure with him straight away into our dressing room."
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