The international transfer regulations prevent his registration from being completed before that date.
The delay is a technical consequence of Éderson joining from an overseas club. FIFA's International Transfer Certificate system — which governs the movement of players between national football associations in different countries — does not process international registrations until the standard benchmark date of July 1. Italy's own transfer administration framework operates on the same timeline.
Éderson has agreed all personal terms and signed his contract. He is, in every practical sense, a Manchester United player. But he cannot be legally listed on the club's squad documentation until the regulatory system allows his registration to be formalised.
The delay has no material impact on United's transfer plans. Éderson will be available for pre-season training from the beginning of July and is expected to complete his medical in the days immediately before the formal registration window opens.
The 26-year-old Brazilian made 45 appearances for Atalanta across all competitions last season.