It depends on our ambitions I think. Signing him to challenge the first team and be depth is what a club like United needs to do. We need to have 1 player per group essentially that can come in for any injury in that grouping of players, or rotating anyone, or resting for fatigue or form (general competition for places), as well as changing things during the game with a stronger bench.
Signing Sancho will give us 4 players to start between 3 positions in attack. Grealish to rotate between Pogba and Fernandes. Matic/McTominay/Fred for the other position at a lower level though. All have enough players on a similar level to get that competition where if someone's not performing, you sub them out or rotate them out and someone else steps in and its what we have to keep building towards to get a squad that has that competition. Loads of games to be played and everyone needs to be on it, no more opportunities to do big rotations, so we need higher level players in the squad to rotate (and essentially play the same amount of minutes as the others, just one to sit out if all are fit and firing).
Of course, Grealish isn't a necessity per se. We have a hole at RW in the attack which is #1 priority, and then next year we'll have to sort out the CDM position, and there are always question marks with our CB composition and fullback group. But as we all saw this restart, we basically can't rest or rotate either Bruno and Pogba, and if their level drops, ours drops. Grealish would stem that drop. So if we could pick 2 signings to make the most difference over a season both in terms of performance and squad development, I'd say it would be Grealish and Sancho. Sorts out the attacking/creative group, while we have depth at the holding mid and defensive positions, just all at a lower quality. But at the same time, our defensive record was good enough over a season that it makes more sense to prioritize the attack and depth in creative midfield.