Your first reaction to words like "Texas Rangers To Place Neftali Feliz On Disabled List" is probably a four letter word, and that's natural. It's probably not that big of a deal, though.
For one, the injury itself is not (yet, to our knowledge) anything serious. Elbow inflammation isn't a torn ligament or anything depressing. Further, pitchers get hurt, that's what they do. The Rangers just went a whole season with their opening day rotation intact, and they have gone a quarter of 2012 before having one go down. In fact, this is the first time all season they've even had to mess with the opening day roster!
Getting that sort of health is awesome, and expecting it to continue would be a bit too optimistic. The fact that the worst injury to a starting pitcher in that time is "inflamed elbow" is pretty good.
Second, it's just not likely to make a big dent on the Rangers' season. Nolan Ryan thinks the injury will have Feliz out a month (or less). In a sixth of a season, even an elite starter (say six Wins Above Replacement) is typically going to be worth only a win or less over a mediocre starter (about two WAR). It could end up being more, but that would just be the luck of the draw. By that same token, you could also luck in to over-performance from the replacement.
Feliz has been good, with 37 strikeouts against 23 walks in 42.2 innings, resulting in a 3.16 ERA and 4.51 FIP. He has not been elite, though. Scott Feldman is no great shakes as a pitcher, but he is a capable Major League starter. The difference between he and Feliz in a single month is pretty tiny in the grand scheme of things. Feldman could be awful, but Feliz could also have had a slump. Feldman could also play over his head.
To go with a more informed estimate, we'll look at ZiPS. ZiPS has Feldman putting up a 4.78 ERA over the rest of the season (mostly as a starter, for some reason, so we'll assume it will be about there); for Feliz, it comes out to 3.70 (sweet!). Big difference, yes? Over a six-start stretch -- a month -- though, that's only going to be about six runs a game; without some bad luck, that's an extra loss for the Rangers at most. A guy who would be a key player in most bullpens fills the empty spot in Texas, and so this DL stint comes out looking like something that will probably knock maybe a win off the Rangers' end-of-season record. Considering how dominant they look in the division, that probably is not a big deal.
Finally, there is limiting Feliz's innings. It's his first year as a starter in the Majors, and his first in a couple years as a professional. Easing him in to the big work load is a smart plan, and this stint will help facilitate that. Feliz was going to lose some starts to Feldman, anyway, for this purpose. As long as the injury ends up being no worse than it looks at this point, there is nothing to wring hands over yet.
Emotionally, it sucks, of course. Seeing Feliz finally start games in the Majors, and seeing him succeed pretty well at it, has been fun. Finding out that fun has to go on hiatus because of health is upsetting, absolutely. It's just not likely to derail his career or the 2012 Texas Rangers campaign.