Last night, the Phillies beat the Red Sox 8-2 to sustain their three-game lead in the NL East. The Phillies were 16-12 when Jimmy Rollins came off the disabled list, and have ripped off a 26-18 stretch with Rollins back on the roster.

Now, you’d think that having Rollins replacing Eric Bruntlett in the lineup would boost the offense, and while it obviously does, the Phillies haven’t been scoring many more runs as a team with the reigning MVP back. It’s actually on the other side of the ball where they’ve been getting much better. The following chart breaks down the Phillies’ performance with and without Rollins in the lineup:

Period      R/G   RA/G
3/31-4/8    4.7    5.0
4/9-5/8     6.8    6.0
5/9-6/16    4.9    3.0

The word you’re looking for is “wow.” Rollins is a better defensive shortstop than Bruntlett is, but not by three runs a game. More strangely, the Phillies were at their best at the plate when Bruntlett, who hit .245/.304/.349 in Rollins’ place, was in the lineup. Chase Utley‘s descent from Ruthian to something a bit more mortal is a big factor there.

It’s the run prevention that’s the truly amazing thing. The Phillies, who came into the season with a so-so pitching staff, average defense, and the same hitter-friendly home park they’ve had for years, have been a lights-out pitching-and-defense squad for five weeks. Just two NL teams, the Braves and Cubs, have allowed fewer runs, 293, than the Phillies have, which may be the biggest surprise in a season full of them.

Wait, maybe this is. The Phillies started the season with 12 pitchers on their 25-man roster. On April 6, they activated Brad Lidge from the DL and demoted Tim Lahey, who hadn’t pitched and was only on the roster because of Lidge’s injury. They have used just 12 pitchers all season long, and have not made a single pitching transaction since activating Lidge. It’s a feat unheard of in modern baseball. Despite having famously fragile Tom Gordon, Rudy Seanez, and Adam Eaton figuring prominently among their pitchers, the Phillies have not put a pitcher on the DL in the regular season. There’s an enormous amount of value in that. Think about the revolving doors in the backs of rotations and bullpens throughout baseball, the performances teams get from the 13th through 18th pitchers in their organization. Certainly, injuries occasionally create space for a top prospect or an opportunity for a pleasant surprise to emerge, but more commonly, when teams have to cycle through arms, runs ensue.

The Phillies haven’t had to do that. They haven’t had to explore “replacement level” in real life. That’s a good thing in any situation, but given the lack of depth the Phillies have at the upper levels, it’s been a godsend for them.

The performance of the staff is another surprise. The rotation outside of Cole Hamels has just been passable, guys throwing 5-6 innings a start and keeping the Phillies in games. The bullpen, however, has been fantastic: a 2.59 ERA with 165 strikeouts in 205 innings, and an absurd 11 home runs allowed. The Phillies have the best WXRL in baseball, rating nearly a full win over the second-place Cubs, and overall their bullpen has been worth nearly seven wins above replacement, a huge figure. It is the single biggest reason the Phillies are in first place, even more important than Utley or Hamels or Pat Burrell.

As great as that performance has been, it’s also one that almost has to change. First, any bullpen getting meaningful work from Traction Action Seanez, Gordon, Lidge, and Ryan Madson is almost certain to have to do without those guys, in some combination, at some point soon. Even if they all remain healthy, their low ERAs and high WXRL are being driven by underlying performances that are not sustainable. J.C. Romero has allowed 15 walks in 26 innings, yet has a 1.38 ERA. Seanez has 14 walks and 13 strikeouts in 22 2/3 innings, and a 2.38 ERA. Chad Durbin, of all people, has a 1.55 ERA.

Even that’s not the biggest anomaly. From The Hardball Times, here are the HR/FB figures for the seven primary…OK, only…members of the Phillies’ bullpen:

Pitcher         2008    2007   2006    2005
Brad Lidge       0.0    14.7   18.0    11.7
J.C. Romero      9.5     6.3    7.6    15.4
Tom Gordon       7.5    15.4   18.2    10.8
Ryan Madson     10.0     9.8   13.4    16.9
Rudy Seanez      0.0    11.7   12.5    11.0
Chad Durbin      2.6    13.9   17.5     N/A
Clay Condrey     7.5     7.5   17.1     N/A

Now, I suppose I could explain the above chart, but it speaks for itself pretty strongly. The Phillies’ bullpen is suddenly allowing home runs at a fraction of the rate that its pitchers have previously allowed them in their careers. It is not because they’ve all suddenly turned into ground-ball pitchers, and it’s not because they moved the fences at Citizens Bank Park. It’s a stone fluke, and while there’s no guarantee that this stone fluke won’t continue right through the end of the season, that’s not the way to bet.

The Phillies have been a great run prevention team this year. The reasons for that, however, are likely unsustainable, primarily a bullpen that has been amazingly healthy, that has allowed fewer runs than its strikeouts and walks suggest, and is posting a wildly out of line home-run rate relative to the flyballs it allows. As the season wears on, all three of those things are likely to revert to normal, which will keep the Phillies from running away in a highly-competitive NL East. There’s just no way these seven pitchers can keep being the best bullpen in baseball.