You're right that the issue is depth, but it's not just about injuries to a couple top guys. The problem is simply having the depth to weather a grueling EPL season, CL, and Cup matches. United's championships are often the result of simply not dropping points that they should take, and IMO that has a lot to do with having such a deep squad.
At the top of the table each of the teams can beat every other one on any given day, the difference usually comes down to what is done against the lower half of the mid table teams.
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