Emotions (and what could really be serious financial concern with trading Manu) aside - should Manu be on the trade block if it means landing an above average big man? A trade involving Manu would obviously be difficult for various reasons (fan favorite, business reasons, is he tradeable due to age/injury...) - but if someone loved Manu enough to offer up a big for him (say Josh Smith for example), would the Spurs have to do it?
Pros: Manu is an expiring contract. Spurs are deep at guard (even though the quality of that depth might be questionable). He makes enough money to land a quality player.
Cons: Huge fan favorite. He is the reason the Spurs have the best bench in the league most years. Possible financial ramifications of trading someone SA absolutely loves.
So if you can get a big man of Smith's caliber for Manu do the Spurs have to do it for basketball reasons? It would be risky because of chemistry, but the Spurs pretty badly need another really good big man.