United showed their faith in the Nigerian by extending his loan until January 31 at a cost of £10.5m, which includes wages and loan fee.
And former Red Devils striker Berbatov thinks the 30-year-old can extend his stay even further.
"The way he linked with the other players, his interaction with the team and everything he did was top quality for the time he was given to play.
"Now, Ighalo has a massive challenge ahead of him to prove that it was the right decision from the club.
"Now that he's staying until 31 January 2021, why not go out there and show the club why he should stay for even longer? Why not fight for a permanent contract?
"He's a great boost for the team and it puts United in a strong position."