That's exactly the point - Diaw's defense was OK at best and he was the best 2nd unit player to defend LBJ. Leading up to the Finals, the Spurs most challenging 3 they had to defend was Harrison Barnes (who is good but certainly not the feature of his team). Next year that might be Durant or Gay that's out there in the latter half of the 3rd qtr when Kawhi needs a breather.
TP, Green, Kawhi, Duncan and either Splitter or Diaw make for a great starting unit (Splitter starts in cases of other teams possessing 2 dominant big men - like Memphis or LA w/ Kaman and Gasol). That unit will do OK vs. most teams.
But, the combo of a not very good decision making PG (like Joseph, Colo or Mills), Ginobili or Neal, Green or Diaw at the 3, Bonner or Pendegraph at the 4 and Baynes at the 5 is pretty turnover prone and has little inside scoring capability. If you insert Belinelli in at PG, you do get some scoring and better decision making but now have horrible perimeter defense.
Backup back court will need to be Belinelli, Ginobili or Neal and a new SF who can play defense and create shots on his own so that Diaw, Bonner, Pendegraph (and sometime Splitter) and Baynes can offer both D and O from the 4 and 5 positions.
Just my thought process but the Spurs REALLY need a backup SF...if DeShaun Thomas ends up maturing/learning the system and capable of decent defense by playoff time, excellent. But I wouldn't count on that if I were RC/Pop. Ronnie Brewer is still available but he has ZERO offense and outside shooting. Luke Walton is intriguing but is fairly old and isn't really stellar offensively or defensively. Mikael Pietrus is my choice for someone who can help out and be happy w/ the remainder of the MLE the Spurs have (if he checks out medically).