A) Need more evidence: every single of these players.
There's nothing that allows us to say Sheppard and Knecht would be "ready to contribute now"... Actually everything, and the mere fact of having zero NBA or pro experience but playing against mostly amateur teens (who will not even become pro for most of them) vs. grown men among the best BB players in the world in the NBA, say the opposite... The fact Sheppard and Knecht can't also be considered as top prospects like Banchero, Chet or even Miller but rather role players, who take much more time to find a place in the NBA, say they certainly wont be ready to really contribute before years, and certainly not more, and actually less, than current spurs players...
Once again we shouldn't make definitive assertments and fantasizing too much about these kids. We just have to watch last year's discussions about the lottery guys and see where they are at now (any year actually). Every young prospect is beautiful in March till june, then the next NBA season starts... Sheppard and Knecht are last year's Gradey . I mean, yeah they could show some interesting things here or there but they wouldn't make this team better in thie first year, actually worth if given plenty playing time.
Simply pick and give consequent playing time to two rookies next season, and it's another crap show.