Alvarez had the infection drained at a hospital Tuesday. He was at Roger Dean Stadium in street clothes Wednesday and walking with the aid of crutches as the Marlins went through their morning workouts.
"Of course it is better that it happens right now than later during the season," Alvarez said through a translator. "I'm trying to do my best to get ready for the season."
The 23-year-old right-hander had been scheduled to start Sunday's game against Washington. The Marlins did not provide a timetable for his return, but Alvarez said he could miss up to a week.
Alvarez said he isn't feeling pain from the infection.
"This is the first time ever something like this has come to me," Alvarez said.
Alvarez went 5-6 with a 3.59 ERA last season. He pitched one of the most suspenseful no-hitters in baseball history on Sept. 29 when he held Detroit hitless through nine innings and the Marlins scored on a wild pitch in the bottom of the ninth for a 1-0 win.