Always see people saying buying 5 or more players in one transfer window is not realistic. Many positions should have been solved for the last few seasons, such as RW, CB, FB. The problem is that we always struggle to sell useless players, we even extend some of their contracts (until now i still get no idea why we extend Rojo, who played only 10 games last season, and 2 games this season).
Let's say First team should have around 23 players, we have almost 10 players (Lukaku, Sanchez, Mata, Fellaini, A. Pereira, Young, Rojo, Jones, Valencia, Darmian) who are either useless , not Man Utd quality, or overrated to their price and wage (I already not count Fred, who still need time to adopt English Premier, Smalling and Bailly). That's why other than the starting XI, we don't have quality players to rotate. Vs Burnley is a good example, and I have never seen a situation of a big team like us so far, keeping dozens of players who are not helpful to the team. So please, offload those players, spend money in the summer (RW : Sancho / Nicolas Pepe ; CM : Ruben Neves / Ndombele ; CB : Koulibaly / De Ligt / N. Milenkovic ; RB : Aaron Wan Bissaka), promote youth players (Greenwood, Chong, Garner, Tuanzebe).
Look at Liverpool, since Klopp came, he almost changed the whole first team, but their net spend is so low, because he sold players who are not suitable for him (Benteke, Coutinho, Ibe, Sakho etc), sold Danny Wards, a goalkeeper who never play for Liverpool for 15M to Leicester City, Solanke 17M to Bournemouth, and we can only wait until the players free go (Mata, A. Pereira, Young, Jones, Valencia, Darmian)