The upper-levels of the Twins minor league system is stocked with relief pitchers performing at a high level. Here is a quick rundown:

Michael Tonkin (AAA, 24)- Tonkin has been arguably the best reliever in the Twins system since the start of 2012. This year in AAA, Tonkin has a 3.08 ERA—nothing special—but his K and BB numbers are excellent: 26 K’s to just 4 BB’s in 26.1 innings. He’s been even better in this department since the start of June: 18 K’s to 1 BB in 18.1 IP. Tonkin has pitched briefly in the majors both this season and last, where he has struggled slightly: in 22.2 IP, he has an ERA of 3.57 to go along with 18 K’s and 9 BB’s.

AJ Achter (AAA, 25)- this is a guy who has drawn plenty of headlines this year, and with good reason. He was promoted to Rochester after 6.2 innings in New Britain, and has since posted a 1.74 ERA in 46.2 IP. His peripherals aren’t staggering—42 K’s and 14 BB’s (8.1 K/9, 3.0 K/BB)—but they are good. It is worth noting, however, that Achter was more dominant early in the season: 24 K: 4 BB in 25.1 IP in April and May, vs. 18 K: 10 BB in 21.1 IP.

Ryan Pressly (AAA, 25)- after a poor showing for the Twins in 2013, Pressly might be a bit of a surprise on this list. His overall numbers very good, but not spectacular: 2.88 ERA, 57 K: 19 BB (3.0 K/BB) in 56.1 innings (9.1 K/9). However, he struggled in April and has been excellent since the start of May. In 45 IP, he has 47 K (9.4 K/9), 9 BB (5.2 K/BB) and a 2.40 ERA. Furthermore, it appears as if the time he spent in the majors led to some major improvement for Pressly, who was never particularly good in the minors. In fact, Pressly is having his best minor league season by far, despite pitching in AAA for the first time. Pressly became a full time reliever in 2012 while pitching in AA, and posted just 21 K:10 BB in 27.2 IP. In other words, he’s striking out 2 more batters per inning while posting a similar BB rate this year.

Deolis Guerra (AAA, 25)- best known for being the only piece from the Johan Santana trade remaining in the Twins system, Guerra missed almost all of 2012 as a result of Tommy John surgery. Strikeouts have never been a problem for Guerra, and that trend has continued this year, as he has struck out 33 in 34.2 IP this year. Walks, however, have been a problem. He has improved slightly in this regard this year, walking 11 batters in 34.2 innings (a respectable 2.9 BB/9) after posting 21 BB in 57.1 IP in 2012 (3.3 BB/9); his BB rate has dropped from 8.6% to 7.6%. Guerra is another guy who has been better since the calendar flipped to May: 27 K 8 BB in 26.2 IP.

Lester Oliveros (AAA, 26)- another guy who missed almost all of 2013 due to Tommy John. Oliveros started the year in New Britain, where he posted a .89 ERA and 36 Ks in 30.1 IP (10.7 K/9); he did, however, walk 14 (4.2 BB/9, 2.57 K:BB). Since his promotion to AAA, Oliveros has 8 K, 2 BB, in 7.1 IP (ERA = 4.91). Furthermore, he has been at his best since the start of June, posting 19 K to just 3 BB in only 14.2 IP across 2 levels (but a ERA = 3.07).

Ryan O’Rouke (AA, 26)- this guy could probably be a lefty specialist for the Twins right now. In 12 IP against lefties, he has just 3 BB and 25 K. You read that right, 25 strikeouts: that’s 18.75 K/9—he’s striking out the opposing batter in 60% of plate appearances. He’s also allowed just 3 hits to lefties: opposing lefty batters are hitting .079/.146/.105 (.252) off him. That’s actually absurd. His dominance of lefties isn’t a new thing: he had 22 K:3 BB against lefties last year in 56 plate appearanes, good for a strike out in 40% of plate appearances (Aroldis Chapman struck out opposing hitters in about 43% of plate appearances in 2012 and 2013, which equated to a 15.5 K/9 during that span).

What it Means Going Forward

I think that most of these players could compete for bullpen positions next spring, if not earlier: they are posting great numbers at upper levels of the minors and are fairly young. Of course relief pitchers don’t make many “Top 100 Prospect Lists,” especially when they’re in their mid-20s. But these players—plus 2014 2^nd round pick Nick Burdi—could form the core of a strong—and cheap—bullpen for the next 5 years.