No wonder foreigners find English difficult. Which is chosen depends on how the speaker is thinking of the people. "The committee were arguing among themselves" sounds more logical than "the committee was arguing among itself" ! But 'the committee has retired' sounds better than 'the committee have retired'.
In the instant case, there's no harm in thinking of Manchester United as the various individual players who represent the club on the field . They will be playing in the premiership. The legal entity which is Manchester United is not physically playing anywhere. It's the choice of the verb that makes the plural apt and appropriate: 'they' are going to be playing. But if you think of it as the entity, the company or the club, then ' it' will be eligible for, or listed in , or a member of, or, if you will, 'playing' in the Premiership.