As of Sunday, Zimmermann, who opted out of the All-Star Game because of the ailment, said the biceps felt much better.
"I feel good. I haven't thrown. The real test is when I throw a baseball for the first time on Thursday, and I'll go from there," Zimmermann said.
If Zimmermann is not ready to go against the Rockies, it would not surprise anyone if the the club called up right-hander Blake Treinen to make the start. Treinen has been outstanding during his Major League stints, allowing nine earned runs in 35 1/3 innings.
Right-hander Tanner Roark will be the fifth starter in the Rockies series.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.