He probably did exactly what you said - tried to pitch through some pain.
Didn’t it strike you as odd that Sale, who was known for being underpaid his whole career, agreed to another extension in which he was clearly going to be underpaid? And did so just 8 months before free agency? If he didn’t know he was hurt then, he clearly strongly suspected his 2019 was not going to help his case for a big money deal…