Janssen, 33, missed the first 41 games with right shoulder inflammation. The reliever, who signed with the Nationals as a free agent in February, made six rehab appearances between Single-A Potomac and Double-A Harrisburg.
The veteran spent his eight seasons with Toronto, recording 90 saves. He went 3-3 with a 3.94 ERA last season.
Solis made five appearances with the Nationals, including his major league debut on April 30. He is 1-0 with a 5.00 ERA on the season.