Howard ruptured his left Achilles' tendon while making the final out in Philadelphia's 1-0 loss to St. Louis in the deciding game of the NLDS last Friday night.
"It's going to be five-to-six months from the surgery until he can play at his accustomed level," general manager Ruben Amaro, Jr., said.
Howard will be immobilized for one to two weeks, and could possibly put weight on his leg in one month.
"A lot depends on how he recovers," Amaro said. "The start of the season could be impacted, but I still hope he makes his first at-bat."