Agree with Duncan here - Achiuwa is not a center, but a PF, who has the capacity, agility and mobility to be able to play small-ball 5, and do it well (not the Rudy Gay kind of "small ball 5). That's one of his main enticing features tbh, not a knack on his size at all. Do agree about his perimeter game being underdeveloped/underwhelming, but to be honest, if Precious had a fleshed out perimeter game, he'd be a top 3 pick in this weak draft. Concessions have to be made... And I think Precious, more than anything, has the tools to excel in his development, even if he isn't quite there yet. He's a willing, and capable, dribbler, especially in the open court but also on half-court offenses, has shown good passes especially when he drives, and has a good-looking form from deep that just hasn't had the reps to be better (1.3 attempts per game if I'm not mistaken). I don't see him developing into a first or second option on offense, but we also don't need him to - his finishing, verticality, PnR abilities, lob threat, rebounding, and especially defense are all things this team needs and will need moving forward. I also don't think Poeltl is a keeper (or at least, not a starter moving forward) so you have to take these things into account. If you can get a floor-spacing C (think Ibaka, but not Ibaka), that's a perfect frontcourt match.
The thing we'd be truly missing is a SF, preferrably a 3.5 forward, along the lines of Tatum or Brown. That's the missing piece - and that's where we tank next season, and go get our future star in next years' lottery picking. I think that's a very solid strategy the Spurs could take in order to quickly put together a very modern NBA team that's young, plays fast and in transition, can shoot from multiple positions, and has a lot of room to grow.