Yonder Alonso's first year in San Diego has been quiet in many ways, but not all of them are his doing. Petco Park was going to deflate his numbers, regardless of how good he is, just as it does for every other hitter. Because of this, Alonso is hitting all of .271/.344/.388 in 130 games and 516 plate appearances this year. Don't be fooled, though, as he might not have rookie status, but he's still a rookie -- and one who has adapted.
On June 27, Alonso's season OPS fell to 668. He had his share of highs in the first half, but they were drowned by lows, and it resulted in a .253/.324/.344 start. Since, though, he's been much more productive, hitting .293/.368/.442, for an 810 OPS that, once adjusted for Petco, is even more impressive than that. Of course, your league doesn't adjust for Petco, meaning all you get is an 810 mark. But there's a good chance, given Alonso's skills, that he'll be much closer to that number than his season-long one in 2013, and that's what you want to remember.
It might not be a perfect fit for first base -- though in NL-only, you'd certainly take it. But Alonso is also a left fielder, at least for the rest of this season. He hasn't lined up at the position, and won't be eligible there in 2013 barring a position change, but if you assume he'll either hit like he has in the second half -- or possibly more with more experience -- then you should be fine with him at first.
Alonso doesn't strike out much, and that gives him an edge at Petco. Yes, balls in play there tend to turn into outs more often, but a strikeout is a guaranteed out, and a ball in play has a chance to be a hit. Even if his BABIP suffers from Petco, by hitting more balls into play, he'll pick up more hits than someone who punches out more often. It's a little thing, but worth remembering.
Alonso is something of a sleeper for 2013 given his strong second half that's masked by his slow start, but take heart in the fact that Petco, while it's hurt him somewhat, hasn't crushed him, especially not during the second half. The samples are small, but keeping an eye on Alonso could turn out to be a positive development for next year's squad.