You are right, Manchester United position in the Premier League could stop them from signing any top player they want since no player wants to play for a team that is struggling with its games, especially when they will not compete in any European competitions. Many players have rejected the team offer because they will not participate in the Champions League next season, if they had made it to the Champions League, many players would not have considered where they ended up this season but no other league to play aside from the Premier League.
However, Mathew Cunha accepted the club offer from Wolves, becoming Amorim first signing, the player didn't care whether the team just played in the Premier League, he accepted the deal. While many individuals assume he joined Manchester United since his previous club only played in the Premier League and survived relegation, he accepted Manchester United offer in hopes that he might play in another competition, perhaps in the next two seasons.
No doubt how difficulty for Manchester United to sign any top players because not participant at European competition make any players more interested joining with other teams can participants at UEFA Champion League or Europe League. For top players, they have bigger ambition for keep existing participants at Champion League for next season if leave their previous team but Manchester United not qualify to European competition for next season.
Mathew Cunha acceptable for joining Manchester United its most luckiness signing, he has good statistic in this season with Wolves have 33 appearances in Premier League success scoring 15 goals and created 6 assists. But signed Mathew Cunha only still not enough for Manchester United if want reach the higher standings position for next season, have sign several top players exactly at defender and midfielder as crucial weakness position for Manchester United in this season.