Ighalo scored four goals in eight appearances upon joining on final day of the January transfer window, only for the season to then be postponed due to the Covid-19 pandemic.
The 31-year-old boyhood United fan's loan was subsequently extended beyond its original May deadline, but Ighalo only scored once more after football restarted.
Ighalo's opportunities have been limited this season, with only four appearances in total including just two starts, but he thanked manager Ole Gunnar Solskjaer for giving him the chance to play for the club he supports.
“It’s so hard to see this dream come to an end,” Ighalo wrote on Instagram.
“But I give God the glory for helping me fulfil this lifelong dream of putting on a Manchester United shirt as a player and represent this great club, it was indeed an honour I will forever cherish and be grateful for.
“To the manager I say thank you for trusting and believing in me when many did not. To my amazing team-mates I will miss you guys, it was always fun and a time I look forward to training and spending time with all of you. And I pray we win the league and FA Cup this year.
“To the best fans in the world we have missed you, but we hear your voices cheering us on from afar, we can’t wait to have you all back on the stands.
"I’m still and would remain a Manchester United fan forever. Once a Red, always a Red. Thank you and God bless.”
Ighalo’s departures leaves Solskjaer with Anthony Martial and Edinson Cavani as his centre-forward options, with Mason Greenwood and Marcus Rashford also capable of leading United’s attack.