Hader (biceps) will throw a bullpen session Tuesday, Brian McTaggart of MLB.com reports.
Hader will take the mound for the first time since he developed left biceps inflammation prior to reporting to Astros camp. While he hasn't yet been officially ruled out for Opening Day, Hader is unlikely to avoid a stint on the injured list. Bryan Abreu appears to be in line to serve as the Astros' closer at the beginning of the season.