Whoscored has Fabinho and Henderson as their worst players this season in their top 12 in minutes and they're 28 and 31, so sadly I think Liverpool spending big on him actually seems plausible. They've brought in Jota and Diaz up top and Konate in the back and Gomez seems to be back and doing well as the backup RB, so investing in a young DM and making Henderson a permanent RCM makes a lot of sense to me.
Then they can sell Fabinho in a couple years or make him a Fernandinho style squad player for a couple.