The Brazilian had been the subject of interest from Barcelona in the transfer window, but the Reds refused to sell the attacker and he is back in training with Jurgen Klopp's side.
While Liverpool had made a fine start to the season prior to Saturday's 5-0 defeat at Manchester City, Moreno sees Coutinho's presence at Anfield as key.
"It is great news," the Spaniard told Mundo Deportivo. "Keeping someone of the stature of Coutinho benefits everyone, the club and the city. He is one of the best players I have played with.
"I do not like to go into details [of Coutinho's failed move to Barcelona]. He is a close friend of mine and he tells me everything, but it's better to ask him."
Coutinho has yet to play for Liverpool this season and missed the Etihad Stadium hammering after agreeing with Klopp to spend more time getting fully match-fit.