It has to do with the moment Boavista is in the table.
They already changed the coach, the other one was younger and tried to play more progressive football, this one is more conservative and using him as a false nine don't know if it's a good option.
Anyway they signed a agreement with the now former Lille owner, and a lot of this players are there because they aren't yet good enough for them, so far it's not working, there are some stupid investors or owners who think you can arrogantly buy clubs and build squads like if you are playing Football Manager.
Honestly don't think it's Angel fault, another one from England who was good last season is Marcus Edwards, this season he is struggling at Guimaraes probably because he had the expectation to leave to a better club or better League.
The club had some problems regarding the transfer market, this to say sometimes it's more difficult to evaluate a player on peripheral leagues, because sometimes the problems they face there are something they aren't used in their countries of origin.
Wouldn't be surprised if Angel has the chance to play at Lille and him having the chance to shine there. Wouldn't mind having him at Sporting though, but yes for now he wouldn't play much at United. But the talent is there.