The pair played together for Watford.
"My pal Ighalo managed to return to England with Manchester United and fair play to him," he said.
“Odion is a proven goalscorer with a fantastic goal record. Wherever he has been, whether that was the Championship, Premier League or Chinese Super League, he always scored goals.
“I am so happy for him because he is an exceptional lad and deserves credit for the devastating impact he has made at Manchester United since joining from China. He is such a humble person and I wish him nothing but the best."