In the tunnel at Atletico's sparkling new stadium on Wednesday, the warmer side of Costa's temperament was on display as he watched his former club take lead.
Television cameras did not spot the anticipated confrontation between Costa and his adversary Antonio Conte — the man he claims ended his Chelsea career by text message back in June.
There was, however, a warm embrace for Conte's brother Gianluca, who is the assistant first-team coach at Chelsea.
This was the first time since the FA Cup final in May that Chelsea found themselves in Costa's vicinity, after a summer in hiding in his hometown of Lagarto.
His former team-mates, several of whom have remained in contact via WhatsApp all this time, joined the fond pre-match farewells.
Despite the repeated transfer requests, despite the threat of legal action, despite the summer strike, many at Chelsea remain fond of Costa.
They remember the striker who catapulted them to two Premier League titles.
View more pictures of Costa during the match below;