The Nigeria international had scored just once in the Premier League this season before departing to the Far East
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The Nigeria international has reportedly moved to the Far East in a deal worth £20 million, though Watford have not revealed the official fee.
Ighalo impressed for the Hornets last season but had fallen down the pecking order under Walter Mazzarri.
Having joined the Vicarage Road outfit from Granada in 2014, he scored 20 goals as Watford won promotion back to the Premier League in 2014-15.
And he continued his fine form into the top-flight, finding the net on 17 occasions as Quique Sanchez Flores's side comfortably avoided relegation.
Under Mazzarri, however, he failed to fire, with his 18 Premier League appearances bringing about just the single goal.
He joins Changchun ahead of the new Chinese Super League season as they look to improve on their 12th-placed finish last term.