Is Duensing headed for a spot in the Twins rotation?

At this point, it seems likely that we'll see Brian Duensing get sent down to Rochester sometime soon to begin stretching himself out as a starter.  Nick Blackburn's leash has been officially extended as far as his start next against the White Sox, and, unless he dramatically turns around his disappointing season in the next two weeks, there is a good chance Duensing could replace him in the rotation by the end of the month.

There would certainly be a substantial amount of support for this move: Twins fans have become (justifiably) fed up with Blackburn, who currently ranks as one of the major's worst starters and begins the second half with a 6.40 ERA.  Duensing, on the other hand, has been outstanding out of the pen, sporting a 1.62 ERA and a sub-1.00 WHIP.  Even better, Duensing responded brilliantly when given a chance to start for the Twins during the 2009 stretch run, going 5-1 with a 2.73 ERA in 9 starts. 

So what should Twins fans expect if Duensing does pitch his way into the Twins rotation?

Well, let's start simple: Twins fans should not expect a 1.62 ERA and a sub-1.00 WHIP.  But you knew that already.  This is obvious if for no other reason than throwing five or six innings is a lot harder than throwing one or two.  More important to this discussion is the fact that Duensing has been extremely lucky this season.  I went over this in a previous post, but just to refresh your memory: among all MLB pitchers who have tossed 30 or more innings, Duensing's .215 BABIP is the 7^th lowest and his 92% strand rate is the 4^th highest.  Neither of these numbers are sustainable and both will go in the wrong direction given enough time. 

Another concern is Duensing's vast splits against right and left-handed hitters:

Duensing vs. Lefties

Duensing vs. Righties

As you can see from the charts above, Duensing has been murder on left-handed hitters, but has posted mediocre to poor peripheral numbers against righties.  My biggest concern with these splits is that Duensing is posting a 5.20 FIP against right-handers even while pitching out of the pen.  If we see his overall rate stats regress as part of his transition to a starter - if, for example, his strikeout rate drops when he starts throwing more than one or two innings at a time - we could see right-handed hitters become an even greater problem for Duensing.

Of course, the problem would only grow as teams stacked their line-ups with right-handed hitters every time Brian took the hill.  As a reliever, Gardenhire has a good deal of control over who Duensing faces.  If he's part of the rotation, opposing managers will certainly work to exploit Brian's biggest weakness.

Look what opposing managers have done to Liriano this year (1.38 FIP vs. LHs, 3.42 FIP vs. RHs):

Francisco Liriano Batters Faced, 2010

Right-handers: 365

Left-handers: 87

Now consider how Duensing would fare under similar circumstances.

Even acknowledging Duensing's flaws, it's hard not to look at him as an improvement over the struggling Blackburn.  Brian's poor splits against right-handers are still better than Blackburn's overall numbers.  That being said, any enthusiasm for Duensing's possible transition to the rotation needs to be tempered by the realization that a) he's been awfully lucky this year, b) starting is a lot harder than relieving, and c) opposing managers will certainly test Brian by forcing more right-handed bats into their line-ups.