Hamels was scratched from Tuesday's start against the Astros due to tightness in his oblique, Evan Grant of the Dallas Morning News reports.
It's unclear if the oblique tightness flared up during warmups or if this is something that has been bothering Hamels in the past. Either way, the starter will not pitch Tuesday against the Astros in a very last-minute scratch. Alex Claudio will take the mound for his first major-league start in Hamels' absence. More news on the injury should become available in the near future.