Most top teams have quality players on the bench, for example, look at Man City and Real Madrid and their embarrassment of riches. We only have 3 quality midfielders for 3 midfield positions. Carrick is now 36 and too old to play at this level and Fellaini is not really a CM but better in an advanced role. We are Manchester United a top tier club and we should have a better squad options given our status. The extra midfielder doesn't just play only in injury situations but can be used off the bench to close out victories and to rotate with the starting players for example when we have two matches per week. Another area we are woefully undermanned is fullbacks. We have an assortment of CBs, wingers, and RBs filling in at LB for the last two years. Compare our situation with Man City who purchased two starting fullbacks and a reserve fullback in one transfer window alone or Conte who buys Tiemoue Bakayoko as a starter and then buys Drinkwater as a reserve. I don't think it's overkill to have 4 quality players for 3 positions.
Real Madrid's all quality players on the bench were considered young talents when bought and develop into the quality they are now. A player like Asensio was bought for less then 5m and wasn't considered any big name at this time, even Barca refused to buy a plus 5m to get him and looks how he developed.
I don't think Man City is a good example, too. They have 3 FBs, 2 on the right and one on the left. That's a very normal depth and we also have it, considering they get rid of all their full backs in summer as well, and in the attack it's normal they have this number of players if they are playing with about 5 at the same time. Aguero, Jesus, De Bruyne, D.Silva + 2 of B.Silva, Sane or Sterling. They are only playing with one midfielder so they need this number of attackers and only one or 2 of them sit on the bench.
Regarding Chelsea getting Drinkwater as a backup, well how is that any different to us having Herrera and Fellaini as a backup to Pogba ? Drinkwater is even less than them in quality. You need to consider that Chelsea has only 4 midfielders with 2 starting ( Kante, Bakayoko, Drinkwater and Fabregas ). That's hardly any quality depth here and if Kante is injured there's no single player in their midfield who can do the same roles as him as efficiently. We have 5 midfielders with 2 of them playing so I really think we have a better depth than them in this position.
Back to what I was saying. You aren't supposed to have players on the bench who can easily replace your most important played when he gets injured. The bench will always be of less quality than the starting players IMO. You can't bring a very good player and tell him to be a rotational option here. If you won't provide him with a starting position another club will easily provide him. Rotational players are always mediocre hardworker that know they can't start for any big team so accept the rotational place, and we already have Herrera and Fellaini in this job.
The other way is to do as Madrid. Bring some interesting very young talents who are willing to accept being rotational ones while they are developing. They may develop to be great players after that or flop. Madrid took the risk several times and always succeed but they were no under pressure as their starting line up is world class anyway.
But if you think you can bring Fabinho and tell him to become rotational option for RB and midfield ? He will never agree. He can move to PSG and will get a starting position immediately behind Verrati and Rabiot so why agree to be a rotational option here ?