There seems to be a misconception as to why people are concerned about Jeff Niemann heading forward. It's not because he's inexperienced or anything silly like that, it's because history is against him.
Since 2004 and including 2009, 92 pitchers have held groundball percentages under 39%, Niemann amongst them. Only five possessed home run per fly ball percentages lower than Niemann, those are:
Chris Young 2007 29% GB, 5% HR/FB
Dallas Braden 2009 38% GB, 5.5% HR/FB
Tim Wakefield 2009 36% GB, 6.4% HR/FB
Edwin Jackson 2009 39% GB, 7.3% HR/FB
Daisuke Matsuzaka 2008 39% GB, 7.5% HR/FB
For reference Niemann sits at 39% and 7.7 HR/FB.
So, of those 92, we have six cases of 39% or less and HR/FB% below 7.7%, and of those six, four are occurring this year. For the 72 pre-2009 cases of pitchers with 39% or less groundballs induced, the average HR/FB% is 11.7. If any of your friends are gullible enough to bet that these 2009ers won't give up more home runs from this point on than they did in the first half, take that bet.
What does that mean for Niemann? Well, it doesn't mean you should expect him to give up 14-15% HR/FB to ‘even things out' over the rest of the year. It does mean that you should expect his HR/FB% to finish closer to the 11-12% league average range. Right now, about 72% of the total batters Niemann has faced have put the ball into play, meaning no home runs, no walks, no strikeouts, etc., and if we expect another 300-350 batters faced, what type of homerun numbers should we expect?
If the trend continues, 244 batters will put the ball into play or homer. If NIemann's 45% fly ball rate continues, that's 110 fly balls. Assuming he gives up a league average 11% HR/FB, there's 12 dingers right there. Knock a homer or two off that total, given that long balls are slightly suppressed at Tropicana Field and that some of those fly balls will be of the infield variety. You're still looking at ~20 jacks, roughly equal to Andy Sonnanstine or Matt Garza's total last year, and in less innings.
Adjusting for ~160-170 innings equals 1.10 home runs per nine. Throw in 5 strikeouts per nine and 3.8 walks per nine then go fishing for comparisons. You get:
Jeff Suppan: 4.9/3.7/1.2
Micah Owings 5.4/4.4/1.3
Vincente Padilla 4.9/4.1/0.8
The FIP on those guys: 5.22, 5.31, 4.67
4.7-5.2 seems like the range we're dealing with, but THT has NIemann's xFIP in the 5.5-5.6 range. The reason his xFIP is higher than the rest of the real FIP is because Owings gives up an equal amount of grounders while maintaining 10% HR/FB, and Suppan/ Padilla are groundballers. Niemann shares the most qualities with the worst of the trio.%.
Unless Niemann starts getting more groundballs, it's not a matter of if his home run rate regresses, but when.