In this last five starts, The Walrus has allowed five, two, six, four, and seven runs, posting an 8.28 ERA over that time. His overall ERA has jumped from 3.25 to 4.03. Hitters have hammered him to the tune of .349 / .415 / .566 over 28 innings. He bottomed out last Saturday with the worst start of his career, lasting just 1.2 innings, giving up seven hits and two walks without striking out anyone.
So what's happening? The bad news for Nationals fans is that this is probably just a matter of Lannan's luck evening out and the "real" Lannan showing up over the course of a full season.
Lannan has for a long time benefited from unusually good luck. For his career, his BABIP against is .280, and especially for a groundball pitcher with one of the league's worst defenses behind him, that's a number that should be around .310 or above. His xFIP has been 1.22, 0.94, and 0.71 runs higher than his ERA in his three MLB seasons.
Over this rough patch, Lannan's key "controllable" rates haven't changed all that much. His K rate over the last five starts is 9.8%. Prior to that he struck out 9.7%. Either way, he's not missing enough bats to be consistently successful as a mid-rotation starter, nevermind an ace.
Some of his rates have declined a bit. His walk rate is up a couple ticks to 9.8%, compared to 7.3% before that. His groundball rate has been 43.7%, which is a significant decline from his season-long 51.3%. But his line drive rate has been just 17.6%, really no change from his season-long 17.9% rate. So he's pitching worse, but not THAT much worse.
The real difference is that his BABIP against in these five games has been .367. Now that's much higher than we should expect, but the fact is that he'll need a couple more conflagrations like last week to get his overall BABIP back to where it will eventually land in the end.
It's another reason why I think the Zimmermann injury is so devastating. Put Jay-Z, Strasburg, and a solid veteran Randy Wolf-type in the front of the rotation, and that would allow you to move Lannan to #4, where he belongs, and you can fill the #5 hole with one of the glut of back-end guys we've stockpiled like Stammen or Balester. That's a pretty competitive rotation. But forcing Lannan into the #2 or #3 role basically guarantees a sub-.500 record.