The Brazilian moved to the Catalan outfit from Santos in the summer and has already formed a strong understanding on the pitch with Messi.
"It is the realisation of a dream, an inexplicable pride, and Messi is a spectacular player, a genius both on and off the field," the Brazilian international told Titan.
"Hopefully I can help him a lot on the pitch. It is an honour to play alongside him."
Messi has spent the last couple of weeks on the sidelines with a thigh injury, but is widely expected to return to action against Osasuna this weekend.