Age 24 is not too old to breakout in AAA. Mangini's .873 OPS in that league isn't shabby, and was comparable to what Smoak did. Mangini's 1 year older than Smoak. If his swing is indeed corrected to allow him to drive the ball now, he's got big-league potential. But here?
With Smoak at first, and Figgins at 3rd, I don't really know how Mangini cracks the lineup. We have 87 guys vying for LF and DH in the next coupla years (Saunders, Chavez, Tenbrink, Poythress potentially, C. Peguero, Halman, Mike Wilson I guess, Carp, etc).
That's not a shuffle Mangini wants to try to win with the game he's shown so far. If you want to play LF or 1B long-term and you don't plan to take walks you need to hit like Kendry Morales not Howie Kendrick.
Or even like Jorge Cantu. The Marlins played him mostly at 3B. They're not big on fielding there in Florida. Of the 153 players Fangraphs lists as having spend ANY time fielding at 3B in 2010 to get a UZR, Cantu ranks 147th. He posted a -8.9 UZR. At his career for 3B he's an amazing -31.4. His fielding % for his career is an abysmal .923.
He should NOT be playing third. However, his bat is average-to-terrible at the other positions he could play. He's at least mostly-neutral with a plus-bat at 3B and the worst glove in the league.
David Wright can screw up some plays at 3B because he clubs like a maniac there. Alex Freaking Gordon got moved off the hot corner because he couldn't field and wasn't hitting enough in the pros to keep him there.
So IMO Mangini needs to improve on last year both offensively and defensively to get his shot here and force Figgins out. He could be an adequate LF bat among the shuffle of possibilities, or a bad 1B one behind Smoak, but I'd rather he be a good 3B bat. Smoak is a pretty big target at 1B - try not to miss him.
If he can't do that, then the odds of him staying aren't good, and his pro ABs are gonna come somewhere else.
~G