Pep Guardiola needs a midfielder, that's for sure. He bought last summer Ilkay Gundogan but he got injured and has to rest until next season.
Even though the club has other players for this position like Fernando, Fernandinho, yaya Toure and Fabian Delph, they want to sign new player, as some of the current ones could leave them at the end of the summer.
Therefore Rafinha would raise the squad's quality. The Brazilian isn't getting enough playing time at Barca and could follow his brother's career. Thiago is currently shining at Bayern Munich after having left the Camp Nou.
Even if he has played this season almost 1000 minutes, he has had time to score six goals and dmade four assists in 22 matches. His current value is supposed to be €15 million high but obviously he wouldn't leave them for that little money.