How is he meant to achieve this? He will be on the bench for 80%+ of the games we play. The only way he gets any sustained time in the team is through an unthinkable Rodri injury.
Look at Ake, the very same arguments were made for us signing him. For the 1st season, he was dump. Last season he was far better but still backup. It has led to transfer speculation around him being too good to play just a dozen PL games a season and him wanting more game time. He either stays and has a repeat of last season or he leaves. He will not be replacing any of the 3 CBs currently in front of him (unless they get injured).
How will the Phillips situation play out any differently? If he has a 1st season like Ake, the £50m price looks absolutely ridiculous. If he plays well in his limited opportunities, he will be then linked with more game time elsewhere.
We have to bring players in who have the capability to become 1st choice in the position they play, whether that be straight away or in a couple of years. You don't pay £40m+ for players who will never be anything more than a backup. If Phillips was out of contract, or available for a pittance, by all means bring him in, but £40m+ for someone to give Rodri a rest? Nah, use the academy for that.