The 35-year-old has scored 16 goals since joining from Paris St-Germain in the summer.
Speaking to Talksport Mino Raiola said: "There is a big chance that there will be a second year.
"We had already included that in the contract so it is not new for us and Manchester United. Both parties are very happy with that.
"It is more a question of time or when rather than if."