Personally I wouldn't call him the "perfect" replacement for Barnes, unless you're talking about demoting Castle's priority in the offense (which there may be an argument for, but I don't think the Spurs want to do this). Basically, I don't think MPJ is a perfect fit for a 4th/5th option. He's just too good for that (and he won't go for that), but that point that's really what we need.
To me the perfect fit is someone who can shoot, rebound and play decent D but is also ready and willing to play the role that Barnes and Julian are currently playing for us and not complain about it. It's too bad Julian isn't a little bigger because he's kind of turning into that idea PF with the way he is rebounding this season.
Of course it deserves repeating, this is likely the most coveted archetype in the league (and guys who can shoot-rebound-defend are stars, not role players) so we shouldn't feel too bad we haven't found that guy.