Massa, a compatriot and friend of Pato's, posted a photo on Instagram on Tuesday congratulating the ex-AC Milan star on his return to Europe with the Premier League champions.
The post reads: "[Pato] I am sure it will be very good for you my bro! Congrats [Chelsea]."
Pato scored 51 goals in 117 Serie A games for Milan in a six-year spell but was troubled by injury during his time at San Siro and returned to his native Brazil with Corinthians in 2013.
After falling out of favour there he joined Sao Paulo on loan a year later, and has since been linked with Liverpool and Manchester United as well as Chelsea as he seeks a permanent exit.