Correct me if I'm wrong, but he wouldn't actually get here until summer camp, right? By then (well before actually), we'll know what our recruiting situation is, which will then inform our 5Y situation, and the program will probably have a good handle on the annual spring "surprise" transfer or two before we have to make any final decision.
So, yes, we might take Scarlett at the expense of a marginal fifth-year player - at absolute best we're talking Amir Carlisle here - and probably not one at a major position of need like defensive line. And I doubt we'd take him if we really think he won't be able to contribute because of this knee injury.
So if you're asking would I rather have, say, Jalen Brown sitting on the bench for 12 games or Brennan Scarlett who could even start and certainly play a lot but might get hurt in week five because he's had more than his share of injuries, it still seems like a no-brainer. And it's just for one year. He's not sucking up a scholarship long term.
I really don't see the downside here.