Stammen said Friday that no timetable has been set for his return.
''It kind of depends when they go in there what they see and what they find,'' Stammen said before Washington hosted Philadelphia. ''I'm praying for something that's good. But you never know - it could be the rest of the year, it could be more than that.''
The 31-year-old righty didn't allow a run in five games and four innings this season. He went on the disabled list Wednesday for the first time in his major league career.
''My initial reaction was probably like `bring it on.' I got another challenge to try to overcome,'' he said.
Stammen was 4-5 with a 3.84 ERA in 72 2-3 innings last year. He plans to remain with the team during his rehab.