Mackay believes they're fortunate to have signed the former Tottenham defender in the summer.
He told Absolute Radio: "We were in the right place at the right time. You know? I honestly think that, you know, having worked with him. I spoke to obviously plenty of people before getting him. But having worked with him just now, for the last few weeks, I honestly think there's the potential there to be an England star of the next 10 years as a centre back for this country."
Mackay was speaking on Absolute Radio's Rock n' Roll Football Show with Ian Wright. Saturday's from 5pm.
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