LOSC snapped up the England youth international on a free transfer from Manchester United before sending him away to Portugal.
Campos explained to O Jogo: "It has to do with the need for him to complete his training. Angel needs to play 30/40 games in one season and I think that, if everything goes well, he will demonstrate that he is one of the greatest talents in Europe.
"We are talking about a player who was the captain of the English national team, who won the Under-17 World Cup, and who is also, I think, the captain of the England U-20s. He is a player that I had to follow several times live, both in the Manchester United teams and in the youth teams. For me, I repeat, he is one of the greatest European talents of today.
"It is necessary to create conditions so that all this talent can explode. I believe that Boavista and his coach can help him in that sense. He will enter a championship that can grow his individual level so that, in the future, he can enter an even higher and more demanding level."