The former Manchester United striker struck two telling blows in the shock 3-2 triumph.
Mackay beamed: "I thought it was an incredible bit of business on our behalf to get him from Sunderland for £600,000 - and he hit the ground running.
"He gave their back four problems throughout and scored his goals. He has an England cap and, over time, I think could force his way back into that squad."
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