Barcelona intention to find a new striker to replace the hole left by Lewandowski indicates that they will not rely on Vitor Roque next season. Since officially joining Barcelona, he hasn't had much playing time. If Barcelona does not use his services much next season, it would be best to loan him to another club to reduce the player's salary burden and gain playing experience while increasing his experience in European competitions.
I don't really know Alexander Isak, not sure whether this player deserves to be the main striker of Barcelona. However, his performance looks quite well in this season, he could score 19 goals in EPL from 26 matches. He is also still young and has enough experience in some big teams (Dortmund, Real Sociedad, Newcastle). But his market value is quite high, it is €70m. I think Newcastle set a minimum price above €80m. How Barcelona can get €80m? I don't think it is enough with the money got from Lewandowski.
Vitor Roque may be quite skillful but he is lack of experience. I think this may be the reason why Barcelona doesn't want to rely on him as the main striker for the next season. I think Vitor Roque is better to stay for another season in Barcelona. Barcelona has lack of striker now, it is not a wise decision to loan Vitor Roque.