By looking at the MLB.com postseason projections page, you can see how neck-and-neck the AL West race has become, and there's obvious value to winning the division rather than being stuck in the one-game coin flip of a Wild Card Game.
With one of the more talented young outfields in baseball, it wouldn't seem like Gomez would make such an impact for the Astros. But realize what's happened to that group since July 1:
- Jake Marisnick: .204/.232/.296, which is similar to an underwhelming career line of .231/.267/.329
- George Springer: Three plate appearances due to a fractured right wrist that could sideline him through most of August
- Colby Rasmus: .203/.253/.377; he's mired in a bad slump in an otherwise productive season
Rookie Preston Tucker has been impressive, but overall, the Houston outfield in July is just 22nd overall in Weighted Runs Created Plus. While you should never judge a player on a mere month of play, Springer's injury and Marisnick's career track record are real indicators that the team needed some help -- particularly since wrist injuries can hamper a hitter's power even after he's returned.
Enter Gomez, who has been one of baseball's most underrated stars for the last several seasons. After something of a breakout in 2012, he blossomed into a true two-way star in 2013-14, using elite defense, above-average power and quality basestealing skills to rank as one of the five most valuable players in baseball, per Wins Above Replacement.
2013-14 WAR Leaders
1. Mike Trout, 18.5 WAR
2. Andrew McCutchen, 15.2 WAR
3. Josh Donaldson, 14.1 WAR
4. Gomez, 13.3 WAR
5. Miguel Cabrera, 12.5 WAR
Over the remainder of the season, Gomez is projected to be worth roughly two wins more than Marisnick, though that value cascades down the line. Marisnick may not be a dangerous hitter, but he's a plus defender and a threat on the bases, making him a useful bench piece. (His Statcast™ average top speed of 17.47 mph on the bases is third in baseball, minimum 50 tracked plays.) When Springer returns, an outfield of Tucker/Gomez/Springer, with Rasmus and Marisnick in reserve, is quite the collection. Except for Rasmus, the outfield is tied to the team through at least next year, making this more than a stretch-run drive.
None of this is to shortchange Fiers, who has been quietly solid by striking out 349 in 341 2/3 big league innings, and the prospects going back to Milwaukee, particularly outfielder Brett Phillips, will be missed. But for the Astros, desperate for success after so much time in the basement, this is a move they absolutely had to make. If they win the division by only a game or two, you'll know why.