Fun with numbers ...
Lowest batting averaged allowed (50 innings minimum):
1. Koji Uehara, 2013 Red Sox: .126
2. Craig Kimbrel, 2012 Braves: .126
3. Eric Gagne, 2003 Dodgers: .133
4. Carlos Marmol, 2008 Cubs: .135
5. Billy Wagner, 1999 Astros: .135
Lowest on-base percentage allowed:
1. Koji Uehara, 2013 Red Sox: .163
2. Dennis Eckersely, 1990 A's: .172
3. Dennis Eckersley, 1989 A's: .175
4. Craig Kimbrel, 2012 Braves: .186
5. Joaquin Benoit, 2010 Rays: .189
Lowest OPS allowed:
1. Craig Kimbrel, 2012 Braves: .358
2. Eric Gagne, 2003 Dodgers: .374
3. Koji Uehara, 2013 Red Sox: .393
4. Dennis Eckersley, 1990 A's: .397
5. Hong-Chih Kuo, 2010 Dodgers: .403
OK, Koji Uehara has clearly been one of the most difficult relief pitchers to hit in a single season. Throw in his impeccable control and he's also been the hardest to reach base against. He's 4-0 with a 1.06 ERA, 19 saves and has retired 37 consecutive batters entering Monday's action. So, is he having one of the most valuable relief seasons ever?
That's a more complicated question. If we look just at relief pitchers since 1960, the answer is clearly no. Baseball-Reference rates Uehara's season at 3.4 WAR, but that pales to the best relief seasons:
1. Rich Gossage, 1975 White Sox: 8.2 WAR
2. John Hiller, 1973 Tigers: 8.1 WAR
3. Mark Eichhorn, 1986 Blue Jays: 7.4 WAR
4. Bruce Sutter, 1977 Cubs: 6.5 WAR
5. Jim Kern, 1979 Rangers: 6.2 WAR
Of course, the difference here is workload. Gossage threw 145 innings with a 1.84 ERA, Hiller 125 with a 1.44 ERA. Eichhorn pitched 157 innings -- averaging more than two innings per appearance -- and went 14-6 with 10 saves as the Blue Jays' workhorse setup man.
Uehara is the modern incarnation of the closer, rarely entering with men on base or before the ninth inning (he started the year as a setup guy). Because of this usage pattern, it's impossible for a modern closer to create the same value as the relief aces of the 1970s or early '80s. Let's look at the most valuable relief seasons since 1988, Eckersley's first full season as the Oakland closer, according to WAR. Tony La Russa is often "credited" with creating the one-inning closer with Eckersley, although that's a bit of a misreading of history since Eckersley recorded 16 saves that year when entering before the ninth. Really, it's been a slow treak to the one-inning closer, La Russa and Eckersley merely being one step along the way.
Anyway, highest WAR for relievers since 1988:
1. Jonathan Papelbon, 2006 Red Sox: 5.0
2. Mariano Rivera, 1996 Yankees: 5.0
3. Steve Farr, 1990 Royals: 4.8
4. Jeff Montgomery, 1989 Royals: 4.6
5. Mark Davis, 1989 Padres: 4.5
(Interestingly, Farr, Montgomery and Davis were all in the bullpen for the 1990 Royals. They finished 75-86.)
Papelbon, pitching in a higher-scoring era than Uehara this year, went 4-2 with 35 saves and a 0.92 ERA, allowing just eight runs in 68.1 innings. Rivera was John Wetteland's setup man in 1996, and it arguably remains his best season as a reliever as he pitched 107.2 innings that year with a 2.97 ERA.
Overall, at least in the context of WAR, Uehara's season is special but not that special. Since 1988, he ranks tied for 64th at 3.4 WAR, the same total Kimbrel has this year for the Braves. Baseball-Reference's system for evaluating relievers does factor in leverage; it's just hard for closers to compile higher WAR figure pitching one-inning stints and entering in higher-leverage situations.
That said, Uehara does fare well in some other advanced stats. Since closers only pitch in close games, they often fare better than starters in Win Probability Added -- basically, an out in a one-run game is more valuable than an out in a five-run game. Uehara is tied with Royals closer Greg Holland for the AL lead at 4.1 WPA (Max Scherzer is the highest-rated AL starter at 3.5 WPA). Among relievers, Uehara ranks 72nd in WPA since 1988 (Troy Percival's 6.58 in 1996 tops that list).
In the end, I'd classify Uehara's season as a great year -- he's right up there with with Kimbrel and Mark Melancon for my reliever of the year -- but not necessarily historic, despite his stingy totals of baserunners allowed.