I'm not going to put a lot of words and effort into this, because ultimately this entire post could be composed of two lines. Two lines, that tell you pretty much everything there is to know. Those two lines, as they pertain to Jack Cust:
2002-2010: 40% groundballs
So far in 2011: 53% groundballs
Prior to 2011, Cust's highest-ever full-season groundball rate was 42%. He was an established fly ball hitter, as you'd expect of a player with his skillset. Cust had been the very picture of a guy who sits back, waits for his pitch, and swings from the heels. Those guys succeed by putting the ball in the air.
And look at him now. It's barely been a month, but of the 58 balls that Cust has put in play, 31 of them have stayed on the ground. This isn't completely uncharted territory for him - he posted a groundball rate of 50% last August - but then, last August, he slugged .364. This is the whole problem. Jack Cust's supposed to hit for power, and a Jack Cust who hits a bunch more groundballs is going to have a more difficult time hitting for power, as we've seen.
Now, it isn't impossible to hit for power with this kind of groundball rate. Some players with comparable rates in the early going: Jason Heyward, Buster Posey, and Howie Kendrick. But then, Heyward is normally a groundball hitter. Posey is normally a groundball hitter. Kendrick is normally a groundball hitter. Cust is not normally a groundball hitter. It's like when a pitcher who usually throws 93 starts throwing 90. Pitchers can succeed by throwing 90, but pitchers who have always thrown 90 tend to do better than pitchers who aren't used to throwing 90.
Jack Cust is not going to stand a chance of being the Jack Cust the Mariners thought they were signing if he keeps putting half his balls in play on the ground. He still doesn't have a home run. He's recently hit some doubles, which is encouraging, but note that those doubles came on high fly balls. He needs to hit more of those. The walks are great and all, and I appreciate that Cust has a .365(!) OBP, but one can't help but notice that he's being outslugged by Chone Figgins.
Let's hope for a turnaround as Cust gets more comfortable. If you're looking to extend the comparison, Brad Wilkerson hit a bunch more groundballs than usual in his final season, and that's a bad sign. Jack Cust might be done as Jack Cust, and this might be his way of showing it. But given that it's early, and given that the M's are actually winning some games, I'm willing to give him a chance to get this righted. If it goes on for another month or two then I'll really be worried, but for now I'll just cross my fingers for the occasional DH ding-dong.