He doesn't need to become great player. Being a good universal backup for both wings would be enough. And we can easily pay 7.5 - 9 mil for such backup. Firminho went for 41 mil for Christ sake! Vietto for 20+. Deulo can be at least as good as they are if he fulfills just half of his potential. The problem with him right now is that he still has glaring weaknesses in his game and needs playing time to eliminate them. He won't get a lot of opportunities to play in Barca, unlike in Everton.
Firmino is a world class player and 41m euros is cheap IMO. I am not sure Deulofeu would be happy playing a Pedro role for us, considering the oversized ego he has.