I don't think you can get an understudy good enough who is willing to play second fiddle (see; Phillips). Not when Pep has shown that Rodri is the one he's most unwilling (rightly) to rotate.
One thing that's quite exciting, though, is Kovacic's role recently has shown that having somebody next to Rodri has meant he is free to go forward where we know he can be effective too. The only downside (because I like Kovacic) is that MK isn't exactly known for being good going forward.
However, if you get a player who can do both defensive and attacking work brilliantly - like Rodri - then it's a recipe for success because they can rotate throughout the game making it even harder for the opposition to mark because they'll have their own ways of getting into attacking areas.
At that point you're looking at players like Gundo and Bruno at Newcastle. I'm sure there's more but I'm not wary enough of them.
Annoyingly, Rice would've been perfect.