Coutinho and Firmino played alongside one another at Liverpool for two-and-a-half years and struck up a friendship in that time, with the new Barcelona star taking the role of best man at Firmino's wedding to Larissa Pereira in Brazil last summer.
On Facebook, the Reds attacker wrote: "Liverpool is no longer the same without you my magical brother. I wish you infinite success, that you enjoy life and realise all your dreams brother. God is always protecting you and your whole family. It was an honour to play with a magician like you."
Coutinho spent five years at Liverpool but did not win a trophy in that time.