Deulofeu played under Koeman for six months in 2017 before leaving the Toffees and joining AC Milan on loan in search of more minutes.
"Speaking from my experience, I have very little to say about Koeman," Deulofeu told Onda Cero radio. "The six months I was at Everton with him, he absolutely added nothing [to my career].
"I wasn't happy in the team or with him. It was a sporting issue. The coach didn't want me while the [club] president did. I wasn't going to have any minutes with him so luckily, I was able to leave.
"I went to the club president and to Koeman and told them that I wanted to leave because if I didn't play, I wasn't going to be happy.
"Luckily, I went to AC Milan. It's obviously very different, Everton from Barcelona, it's a different world, different players. He is a coach that has character in the locker room. But from what I can say, at Everton it didn't work out and you didn't see [the team playing] good football. We shall see what happens at Barca."
Koeman, who left his role as Netherlands boss to return to Barcelona, has a long history at the Camp Nou.
As a player, he was part of Johan Cruyff's "Dream Team" and scored the winning goal when the club won the European Cup for the first time in 1992.
He returned to Camp Nou as Louis van Gaal's assistant before embarking on a managerial career that has seen him coach in Netherlands, Portugal, Spain and England.
Deulofeu, who left Barcelona in 2018 to join Watford, said it was painful to watch his boyhood club lose 8-2 against Bayern Munich in the Champions League quarterfinals.
"It was terrible," he said. "It felt like they [Barca] were playing against planes. Few times have I seen in a competition as the Champions League is, to see what I saw. It really shocked me. I saw a very weak Barca in the game against Bayern."
Barca are looking to make a major overhaul of the squad, with veteran players expected to depart this summer to make room for the youngsters like Ansu Fati, Riqui Puig, Ronald Araujo, Francisco Trincao and Pedri Gonzalez, a move that Deulofeu considers long overdue.
"I think Barca are years late when it comes to giving the young players a chance, having the patience to allow them to grow in the team," he added. "People like me who have been there know that this should have been done years ago."