Catcher Welington Castillo has declined his player option to remain with the Orioles in 2018, according to an industry source, officially becoming a free agent. Castillo had until 5 p.m. Monday to decide whether to exercise a $7 million player option for next season at the back end of a two-year, $13 million deal he signed with the Orioles last offseason. Considering the 30-year-old Castillo enjoyed one of the best all-around seasons of his major league career, the move was expected. The Orioles are free to negotiate a new deal with Castillo — but as of 5 p.m. Monday he can sign with any other club. Castillo’s 2.1 WAR (wins above replacement) this season ranked the second-highest among free-agent catchers, trailing only Alex Avila (2.7) of the Cubs. The Orioles are likely to staff the position next year internally with top catching prospect Chance Sisco and veteran Caleb Joseph. Minor-league catcher Austin Wynns could also be another potential catching possibility. “We do have some catching depth in the organization,” Orioles executive vice president of baseball operations Dan Duquette said in September. “We’re encouraged with how Sisco developed. Caleb Joseph came back and had a good year. The catcher in Double-A, Austin Wynns, had a solid year. We have pretty good depth at the position.”