Amaro said an MRI revealed that Howard has two ligaments that are affected.
"The good news," Amaro said, "is that it wasn't a severe sprain, and it only affected the lateral side of the ankle, not the medial side."
Howard, 30, was hitting .292 with 23 home runs and a National League-leading 81 RBIs in 104 games for the Phillies this season. He suffered the injury while awkwardly striding back into second base in the first inning of Sunday's game against Washington at Nationals Park.
The Phillies allowed Howard to fly to Miami on Sunday night in preparation for their three-game series with the Marlins, because they initially believed the injury wasn't so significant.
"Obviously it's more serious than I first thought after talking to him," manager Charlie Manuel.
Amaro said Howard suffered "some pretty significant" swelling in the ankle overnight and was flown to Philadelphia on Tuesday morning to be examined by Phillies team physician Michael Ciccotti.
Amaro did not believe the sprain was as severe as the one endured by shortstop Jimmy Rollins two seasons ago.
Still, Howard is the 15th different Phillies player to land on the disabled list this season. Others include Chase Utley -- who is expected to have the splint removed from his left thumb in several days -- Shane Victorino, Rollins, Placido Polanco and Joe Blanton.
Without Howard, the Phillies' bench will be one man short Tuesday night. Cody Ransom replaced Howard in the lineup.
"We'll get through it," Manuel said, accustomed by now to adjusting to adverse situations.
Said Amaro: "Injuries are a part of the game you can't control. We'll see how it plays out."
Mayberry, 26, was hitting .258 (99-for-383) with 12 home runs and 54 RBIs in 99 games for Lehigh Valley this season. He's expected to arrive before Wednesday night's game to join the Phillies.
Amaro believes Mayberry will add some versatility, since he has played some first base in addition to the outfield.
Charlie Nobles is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.