Do you buy Diaw at the 5?
By and large, yes (I had a short answer to this and then I went on and on ...).
Summary -- below average individual defender, average team defender, very poor rebounder + creates a mismatch offensively on almost nightly basis. Large offensive value.
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As an individual defender, Diaw will be below average but usually serviceable enough. He does a good job fighting for post position and defending the post from thereon out + Diaw will be able to get out on jump-shooting bigs (10-15 feet jumpers or a perimeter big like Okur) ... but opposing centers will still be able to shoot over the top of Diaw quite regularly.
So expect opposing centers to shoot a consistently high percentage (mainly of hustle style baskets) ... but only teams with top offensive threats at the position will be comfortable enough attacking Diaw repeatedly in the post. A couple of teams will go at him 2-3 times but only a small number, with top offensive players at the position, will attack Diaw repeatedly and force you to make a decision.
Against those players, you are either just going to have to live with it (either letting opponent big score or by double teaming and opening the door for someone else to score) or be forced to put in Mozgov who is also a below average defender (man-to-man and team D) but does provide a bit more size and length to combat against. Miami also have extremely quick and capable team defenders on the wing who allow you to make very effective double teams in the post and can get back to contest their man at the three point line well enough.
In terms of help defense, the duties of a center or different than a PF ... it is important that they provide a deterrent in the paint (rim protection, shot altering. Hence, Diaw's value is considerably lower at center than at power forward due his lack of shot-blocking ability but he is still a solid help defender.
He'll make high quality defensive rotations both on the interior and on the perimeter, excellent pick and roll defender, very good at closing out on shooters, very good transition defense and he'll likely draw a few charges. However, his lack of shot-blocking will allow teams opposing teams to proceed to the rim relatively unimpeded. Josh Smith's shot-blocking will help here but not enough as you can see in Atlanta (and Horford is a much more intimidating shot-blocker than Diaw will be).
Diaw's defensive quality, overall, is a lot lower than it is at power forward. His man-to-man D is considerably weakened and his lack of shot blocking goes from a minor issue to a major weak point.
Overall, defensively, I would consider him a slightly below average defender. A below average man-to-man defender and an average team to slightly above average team defender.
Someone like Joel Anthony who is a shot-blocker would have given you a lot more defensive contributions.
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Tangent ...
The one thing that I would have loved to have seen Miami do defensively, with Diaw at center, is to have a big point guard who can switch more readily onto other players. The defensive ability of the point guards in team defense is going to be a lot more important due to the undersized front-line.
Miami are going to have to help a lot in post which will lead to a large number of defensive rotations and switches. It would be best to have a big point guard who can handle some switches onto wing players. Neither Jason Terry or Ty Lawson are capable of doing that ... which will lead to more help defense (when they are caught in a bad mismatch) and more high percentage scoring opportunities for opponents.
This lack of size at the point negatively effects Miami's ability to play high level team defense with an undersized non-shot blocking center.
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As a rebounder, Diaw is very poor. Opponent big men are going to have a big advantage on the glass and regularly out-rebound Diaw by 4-5 boards a night. Strong rebounding wings like Wade and LeBron (also George + Daye) will give him some protection.
Offensively, he creates a mismatch for you on almost a nightly basis. His jump shooting ability (biggest key), ball-handling (second biggest problem) and passing is going to be very difficult to defend against for most teams in the league.
Diaw's offensive ability is going to force opposing team's big men away from the paint and open things up for Miami's slashers. That is a major problem for opposing team's defense. He makes Miami much harder to defend against.
For example, you pull Dwight Howard away from the hoop and Orlando's whole defense falls apart because without him they have nobody to protect the paint + have several weak individual defensive players. Dwight Howard is also hugely uncomfortable defending big men who can face up and attack him off the dribble (like Bosh who has been very successful against Dwight) ... and if helps on the interior, your wings can kick it out to Diaw for wide open 18-20 footers all night long. Very tough to defend against.
