Yes he was. He only pitched 158 innings in 2018. Didn't pitch at all from Aug 13 - Sept 10 due to a shoulder injury.
From Over the Monster:
Sale suffered a shoulder injury in late-July, then came back for one amazing start in August — one hit and 12 strikeouts over five innings against the Orioles — before missing another month with the same ailment. Once he got back, Sale made four starts in September to finish the year, but they weren’t typical starts. Instead, he was being eased back in to get ready for the postseason. It was the right move for the team, but it did cost the southpaw a legitimate chance at the Cy Young.
Then, the postseason rolled around and the team was hoping to get the midsummer version of their ace on the biggest stage... For his standards, this was a negative as he never recorded more than 16 outs in a start and finished the playoffs with a 4.11 ERA. The team won four of the five games in which he appeared, but he wasn’t the dominant Sale we wanted to see.
The ace is set to hit free agency after the 2019 campaign, which means we can be expected to hear a whole lot of extension talk throughout the year, unless it gets done sooner. He has expressed an interest to get something done, per reports. As far as him on the mound, his shoulder should recover through rest this offseason, and the team will be sure to take it easy on him in spring training.