United boss Ole Gunnar Solskjaer said after the 1-0 friendly win over Kristiansund on Tuesday that Bailly had undergone knee surgery following the injury he sustained against Spurs in the International Champions Cup.
"Eric has had an operation and will be out for four to five months," Solskjaer said. "Hopefully we'll have him back around Christmas time.
"It was positive news from the surgeon, he had the operation this morning. He was positive it was all repaired and that he'll be fine."
Bailly was brought on at half-time in Thursday's ICC match, but pulled up with the knee problem just nine minutes after coming on as he tried to prevent Heung-Min Son from scoring.
@uthadebayo on twitter