The 30-year-old striker suffered a hairline fracture playing in a behind-closed-doors game for Rotherham, who he has scored twice for in five appearances since joining on a season-long loan from fellow Championship side Norwich.
"Regretfully the player will be out injured for at least six weeks with a fractured heel," Millers boss Steve Evans told the club website.
"The player picked up this injury in a behind-closed-door match, at a point when no player was near him and he twisted to make a run in the opposite direction.
"The player was X-rayed straight after the match which showed no damage but a further scan on Friday morning diagnosed the hairline fracture."
Becchio will now have to miss this Friday's match against former club Leeds United and probably won't be available for selection until December.