The Nigerian striker's loan at Old Trafford is due to come to an end this month and, as expected, his stay will not be extended.
Ighalo is set to return to parent club Shanghai Shenhua, though has previously talked up a move to Major League Soccer in the U.S.
After playing a bit-part role last season, Ighalo's time at United was effectively ended by the arrival of Edinson Cavani to the club in October.
Writing on Instagram, Ighalo said: "It's so hard to see this dream come to an end. 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.
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"It was indeed an honour I will forever cherish and be grateful for."
Ighalo was a shock deadline day signing for United last January, arriving on an initial six-month loan deal.
That was extended by a further seven months after his decent initial contribution to United.
He scored five goals in 19 appearances last season (only seven of which were as a starter), including a brace in the FA Cup win over Derby and a thumping strike against LASK in the Europa League.
But his chances were limited this season as Ighalo featured on just four occasions for the club, with his last appearance coming as an 89th minute substitute in the defeat to PSG at the beginning of December.
Nevertheless, Ighalo showed his gratitude to boss Ole Gunnar Solskjaer for the opportunity.
"To the manager I say thank you for trusting and believing in me when many did not," he continued.
"To my amazing team-mates, I will miss you guys. It was always fun and I looked 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 back in the stands."
He added: "I will remain a Manchester United fan forever. Once a red, always a red.
"Thank you Manchester United and God bless."