Coutinho has recently lost his place in Ernesto Valverde's starting line-up but Busquets believes he has a role to play this season.
"We all want to play and there's tough competition," he said after the 2-1 first-leg loss.
"There are a lot of us in the midfield but only three positions to fill.
"It's hard to come in when you don't usually play because you lack confidence and rhythm but I think Philippe is going to help us this season."
Barcelona are determined to go on and win the cup as they continue to challenge on all fronts.
"We are going to fight in all competitions.
"[Copa del Rey] has been an important competition for Barcelona.
"It is a tough month with a demanding schedule but you have to give players a break sometimes."