There is no way he can be the starting PG next season.
It's obvious that the best path towards long-term success is trying to find the pieces that fit with Porzingis. Willy fits alongside him offensively for sure, but defense is the question. I think it's worth letting them work together as starting PF/C, but in order for it to really work, they need to be complimented with defense, shooting, and ball movement. D Rose does none of the 3. As a 6th man, sure, bring him back for 6th man money. He can be great in that jamal crawford/lou Williams role if he embraces it.
Looking at our most likely draft scenarios, we probably finish in the 5-8 range. Let's assume we don't get #1 or 2. If we got lucky enough for 3 or 4, you draft Jackson and plug him into the 3 as a point forward/lock-down defender. He's probably the next Paul George. You trade Melo to LAC, with Rivers coming back as the significant piece. Starters: KP, Willy, Jackson, Lee, Rivers, 6th man Thomas. That surrounds KP and Willy with shooter, defenders, and ball movers.
If you draft Fox, which is more likely, you start him at the PG right away. KP, Willy, Thomas, Lee, and Fox, 6th man Rivers. Great defenders at all positions with Rivers coming off the bench as the ideal 3rd guard. If you draft Monk, you probably put Rivers in the line-up at PG and come off the bench with Monk until you figure out what his natural position is. Same with Ntilikina.
Just my 2 cents. I want to build towards the future with guys who fit well together. Willy is essentially a non-shooter at this point in his career (although he looks like he will get there), so we need the 3 other guys on the court to be able to make shots, defend, and pass. That is the only way to build moving forward. I don't see Rose fitting that equation, at least as a starter. Can't afford to have a position on the court that doesn't give you shooting, defense, or ball movement.