I explored some more with BP's statistics. This time I looked at the quality of batters each pitcher has faced. 

The pitcher who has faced the highest quality of opposing batter is Devin Smeltzer. I found it interesting that opposing batters have fared worse by OPS against Smeltzer(.756) than the rest of the pitchers they have faced (.780). Tyler Duffey has dominated this year. He hasn't faced near the quality of batter (.731) but with a .582 OPS against he has the biggest differential on the team.

I don't know what to make of slash stat data at this sample. The opposing batter data set would be huge but the data set for the individual player maybe small. While I am very certain that Smeltzer overall has faced a more difficult set of hitters than Duffey I don't know what to make of their individual opposing OPS number. In any case I find it encouraging to be on the positive side.

Of the pitchers that could be eligible for post season there are 11 who have a better OPS against than the opposing pitchers of their batter set. This set is order from biggest positive differential to least.

Tyler Duffey
Trevor May
Sergio Romo
Taylor Rogers
Jake Odorizzi
Randy Dobnak
Ryne Harper
Jose Berrios
Zack Littell
Devin Smeltzer
Brusdar Graterol

Harper has performed much worse recently. I don't think I would have him on my playoff roster. The other 10 would be on my roster.

The remainder needs to come from pitchers who have performed worse by OPS than the other pitchers that have faced the same batters. Stashak, Perez and Gibson are close to even but Perez and Gibson have not performed well recently either. The only other pitcher in consideration is Thorpe who has been hit hard by opposing batters. I guess I would go with Perez and Stashak but treat Perez as more of an opener with a really short leash second time through the line up.

Two other Twin pitchers posted a positive this season. Mike Morin and Michael Pineda. Pineda really let us down.

Maybe another factor to consider is, like with Smeltzer, when he is brought in and facing a good hitter; he is usually brought in to face a left handed batter and that may be part of why he does well against good hitters.  

But, nothing against Smeltzer, just wondering if that needs to be factored in; since lefty on lefty, everything else being considered, is an advantage to the pitcher.

Interesting post, thanks - some food for thought!

Maybe another factor to consider is, like with Smeltzer, when he is brought in and facing a good hitter; he is usually brought in to face a left handed batter and that may be part of why he does well against good hitters.  

But, nothing against Smeltzer, just wondering if that needs to be factored in; since lefty on lefty, everything else being considered, is an advantage to the pitcher.

I should have looked at that and now that I did I was surprised by the data.

Smeltzer has faced right handed batters in over 75% of the plate appearances against him. That is much higher than the team split of 59% right handed batters. I guess it isn't that surprising since most of the plate appearances against him have come in starts where the other team stacked the line up with right handed batters.

I was surprised that he has been much more effective against right handed batters this year who have posted a .721 OPS against him. Lefties have hit him much harder at an .869 OPS.

I think the answer, with regards to Astudillo is right there in your description of Barnes as a pitcher. Astudillo doesn't strike out, so if the imperative is to make contact, it's hard to go wrong with Willians. A high k pitcher is a perfect opportunity to use him if the goal is simply to put a ball in play, given his unique approach