The problem with respect to Montero’s ability to mitigate the problem of A-Rod’s contract is that offensive value isn’t purely fungible, because that offensive value has to be attached to a specific defensive position the player is at least minimally capable of playing. If Montero were able to be even an adequate catcher, he really would have been able to pick up the slack for A-Rod a bit, because his offensive production could have been slotted in at a different position. But if he can’t catch, and if we assume that A-Rod isn’t going to be benched or released when he’s no longer capable of playing the field regularly, then any way you slice it, having Montero on the roster would give the Yankees three players for two positions.
Now, that’s not to say there couldn’t be ways around that. Most obviously you could have just continued putting A-Rod at third so long as he could stand there and swing a bat. You’d have to account for the lost run value on defense as A-Rod’s skills diminished at the least, and more likely you’d have to figure you’d be getting little more than replacement level contributions from the position when A-Rod was physically unable to play, but perhaps Montero could make up for that with his offense alone. Probably not, but it would be possible. Of course, this doesn’t account for what the pitching staff would look like at this time either, which we have to keep in the back of our minds, at least.
In a perfect world, the Yankees wouldn’t have made such a ridiculous commitment to Alex Rodriguez. But they did, and now that reality simply has to be dealt with. A-Rod is going to be the primary DH for this team sometime in the not to distant future, and considering that they have to pay him either way, that’s probably for the best. The Yankees would have been most fortunate if Montero had been able to develop as a catcher, but barring that, they got about as lucky as they could have hoped to in managing to trade Montero (a long-term supply glut) for Michael Pineda, another young player with a more obviously role on the roster for the foreseeable future. The added value they’ll get from the starting rotation thanks to Pineda actually will mitigate the A-Rod problem somewhat, and if the Bombers manage to get their hands on a 2-4 win third baseman to replace A-Rod, they will have basically spun out the best case scenario for dealing with Alex’s (expensive) decline.