The loan deal with his parent club Shanghai Shenhua officially expired on Sunday night.
The Sun says a deal to extend his stay at Old Trafford has not yet been agreed.
United had doubts over the cost to keep the 30-year-old striker beyond the current terms.
Ighalo swapped the Chinese Super League for the Premier League in a shock deadline day loan deal back in January.
The Nigerian made a personal plea to Shanghai Shenhua to let him finish the season at Old Trafford, before returning to China.