The power-hitting first baseman was injured on the final play of Philadelphia's season-ending loss to St. Louis in Game 5 of the NL division series Friday night, falling as he ran out of the batter's box on his groundout.
The team said Saturday night an MRI revealed a rupture of Howard's left Achilles tendon.
"He cannot have surgery until the swelling resolves, and it has not been determined who will perform the surgery," the Phillies said in a statement. "Recovery time won't be known until after the surgery is complete, and there is no guarantee he will be ready for spring training."
Injuries of that type usually require at least six months of recuperation. When soccer star David Beckham tore his Achilles tendon on March 14 last year, he was sidelined until Sept. 11.
The voluntary reporting date for position players is in late February, and the Phillies open at Pittsburgh on April 5, two days shy of six months after Howard was hurt.
Howard, the 2006 NL MVP, batted .253 with a team-high 33 homers and 116 RBIs this season to help Philadelphia win its fifth straight NL East title. The Phillies set a club record with 102 wins.
Howard was 2-for-19 (.105) with a homer and six RBIs in the NL playoffs against the Cardinals. The slugger went 0-for-4 in the series finale as Chris Carpenter pitched St. Louis to a 1-0 victory, finishing off a stunning upset of the Phillies.
Philadelphia also said left-hander Cole Hamels will have loose bodies removed from his left elbow on Friday and another surgery a week later to repair a hernia.
Right fielder Hunter Pence and second baseman Placido Polanco will both have MRIs to evaluate their hernias. Polanco probably will have surgery and the scan will help determine what Pence needs.
First baseman Ross Gload probably will have arthroscopic surgery on his right hip after it is re-examined by Dr. Bryan Kelly.