The Nigerian striker left Vicarage Road on deadline day to join Changchun Yatai in a £20million deal.
Watford manager Mazzarri said the sale of Ighalo was not his idea.
"Ighalo is a very nice person. He's very professional," the Italian said after the 2-1 win at Arsenal.
"I wish him good luck but sometimes there are decisions which go over the manager's head.
"Sometimes the club and the player make the decisions and this time it was like this.
"But I want to wish him good luck because firstly he's a great person but also a great professional."