Indeed, if Amorim does not get the desired player, surely United will not experience much development. What happens may not be much different from what they achieved this season. Competing with stronger teams will be very difficult for United.
Let's wait and see what they will get in the transfer market this time. Unfortunately, they did not win the Europa League, if they had the situation might be different.
Losing the Europa League final match results in many terrible consequences for them like lack of attraction to star players in the summer transfer season, and no chance of playing in UEFA League next season even Conference League. This means their club finance will be affected a lot next season, and they will have unwanted difficulty in squad developments in the summer.
What done already done, unchangeable so now it's time to move on and focus on doing as best as they can in the summer for preparation of a next season. If they do things bad in the summer, they will have bad preparation for a new season that won't lead to anything promising.
Amorim's steps will be much more difficult if he does not have a number of the players he needs and it will be quite difficult to compete with other top teams if he relies on a number of players he currently has. Changes need to be made and this does not reflect Man United as one of the strong teams in the Premier League because they have experienced quite a big decline.
Manchester United have enough money for building a good squad but they did not do well on transfer market and wasted a lot of money there while their squad quality was not improved in many seasons. First of all, improvements must come from players in the club, and next is a responsibility of head coach to do his management well in training and matches.
I believe Ruben Amorim is not a bad coach but he will face with a next very difficult season and if the club want to build up a long term development future with him, a next season target would be a position in top six. That would be an acceptable target for them by focusing on rebuilding their squad for future like 2 or 3 more seasons with Ruben Amorim.