Machado, who had left knee surgery Oct. 14, had visited his Los Angeles-based surgeon Dr. Neal ElAttrache on Thursday and was given the go-ahead to resume all activities.
However, Showalter doesn't want to rush the 21-year-old All-Star, who injured his knee at Tampa Bay on Sept. 23.
"The thought of him starting the season on the active list is not realistic," Showalter said.
Machado was not available to discuss his injury on Sunday. On Saturday, Machado said that he didn't want to put a timetable on his return.
"He knows the level he has to perform at to be in the big leagues. We want to get it right the first time," Showalter said.
Machado will stay in Florida for extended spring training games, and then possibly play in some rehab games in Frederick, Bowie and Norfolk.
After Machado's surgery, a 4 to 6 month recovery was forecast.
Showalter said that he expected Machado to play again next month.
"This isn't a failure by any stretch. I think you're still going to get him sometime in April," Showalter said.
The Orioles will have to decide on a third baseman in Machado's absence -- and perhaps a second baseman if Ryan Flaherty is moved to third.
Showalter, who made his decision early Sunday afternoon, told Machado during the fifth inning of Sunday's game. He said Machado accepted the decision, but is eager to play.
"He likes our guys, but he's tired of watching," Showalter said.