Jon Lester suffered a hamstring injury running the bases during the Cubs home opener on April 8. He hit the injured list a couple days later after originally thinking he could make his next start.
This afternoon, he is being activated to start against the Dodgers:
The option of Tim Collins is a bit of a surprise, as he had pitched reasonably well in four appearances for the Cubs since his recall April 10. The Cubs apparently are happy enough with Kyle Ryan and Randy Rosario as lefties in the pen, at least until Mike Montgomery returns.