Costa and Conte had a tempestuous relationship at Stamford Bridge despite the striker's goals firing the Blues to the Premier League title last season.
Costa returned to former club Atletico Madrid when their transfer ban was lifted in January, but he was a keen observer of Chelsea's trip to Camp Nou on Wednesday.
Watching on at home, Costa posted a shot of a team news graphic to his Instagram stories.
But there was one omission from the Chelsea squad list, with Costa scribbling out Conte's name.
Costa, 29, has thrived on his return to Atletico despite a string of niggling injuries, scoring six goals for the La Liga side.