Ruthless.. minimum requirements. It's exactly why Man Utd is where it is. You should stick to a plan, regardless of the results. You are where you are because you signed Moyes (who simply wasn't good enough), then Van Gaal (who was playing boring football, but actually building something), then Mourinho (because he was supposed to always win, but a total different manager to Van Gaal therefore totally new plan) and then Solskjaer (who basicly had no plan).
If you want to get somewhere you need a plan and you need to stick to it regardless of the results. You are in a massive rebuild and not in a great position in terms of players being available to you. Elite players and top prospects are not joining a team that needs a rebuild and don't play Champions League. Klopp needed two years to get to top 4 and 4 years to challenge for the title and that was in a time where the top teams of that period were declining (bar City), whereas right now Liverpool and City are miles ahead and don't look like declining within the next three years.
United could challenge top 4 next season, but it also can easily happen that United don't. New manager with a new philosophy needs time to adjust. The squad needs to be adjusted which may not be possible enough in one or two windows. The difference between getting a lucky streak or an unlucky streak in the season can easily add up to 15-20 points. This can be the difference between 3rd and 10th place. If you're finishing 10th because of a significant unlucky streak you should be able to recognize that, not panic and stick to the plan. It is a realistic possibility.
Next season won't be perfect, the game plan won't be executed perfectly, players will probably make costly mistakes here and there. That's all part of a new project. If things go well and you'll get some luck you can challenge for 4th. If the changes take more time than hoped for and United also get a bit a an unlucky run, United can easily end up somewhere around the 8th place as well. Doesn't mean you should change the plan, because as mentioned that is exactly why United is where it is right now.