Self-proclaimed "really bad pinch-hitter" Miguel Montero hit a monster pinch-hit grand slam

Miguel Montero is a bad pinch-hitter. Scratch that, Miguel Montero is a really bad pinch-hitter.
I am well aware that those are very rude comments. If you just read those two sentences and immediately fired off an angry e-mail, I totally understand. But please note, those are not my words. They are the words of the man himself.

Hey seriously I'm really bad pitch hitter lol

— Miguel Montero (@miggymont26) April 20, 2015

That's right, the Cubs catcher who just hit an eighth-inning pinch-hit grand slam to break a 3-3 tie in Game 1 of the NLCS is a self-proclaimed "really bad [pinch-]hitter." "Seriously."
Seriously, I'm not sure that I believe you, Miguel. The pinch-hitting stakes do not get much higher than they were for Montero Saturday night. Bases loaded, two outs, tie game, eighth inning … that's just about as intense as it gets.
There's a stat called Leverage Index (LI) which quantifies the importance of a single plate appearance to the outcome of a game. The LI for an average at bat is 1.00. The LI for Montero's at-bat? 5.38! The only higher LI a pinch-hitter has faced during this postseason was when Wilmer Difo pinch-hit for the Nationals in the bottom of the ninth with two on and two outs and his team trailing by one run during Game 5 of the NLDS. The intensity of the situation Montero faced was tremendous and he responded by doing this:

Until tonight, a postseason pinch-hit grand slam was as rare as a postseason no-hitter -- occurring just twice in Major League history. The only previous postseason pinch-hit grand slams? The Reds' Mark Lewis off the Dodgers' Mark Guthrie in Game 3 of the 1995 NLDS and the Yankees' Ricky Ledee off Boston's Rod Beck in Game 4 of the 1999 ALCS.
Even in the regular season, these don't come around very often. The Cubs have had just five pinch-hit grand slams since 2000: Roosevelt Brown (9/19/00), Julio Zuleta (6/5/01), Michael Barrett (5/28/04), Derrek Lee (5/19/07), Mike Fontenot (5/7/10), and David DeJesus (5/11/12).
So, Miguel Montero is a really bad pinch-hitter? I don't think so.
In his defense, he sent that tweet after he grounded out in a pinch-hit appearance. Also, throughout his career, Montero is batting just .190 in 105 career at-bats as a pinch-hitter. Considering he's a career .259 hitter, maybe Montero's tweet didn't completely miss the mark. He has not performed as well historically in a pinch-hitting role which just makes the events of Saturday night's game all the more remarkable.
"I couldn't really figure it out the last couple at-bats," said Montero in a post-game press conference. " And today obviously after the first pitch, I'm looking something middle, middle in, and he threw me that slider. It was a really good pitch to hit, and I missed it. And in my mind was like, oh, my God, I missed that. That was a perfect pitch to hit ... But to be honest, in the back of my head I was like, I want that slider back, because it was such a good pitch to hit. And I guess he heard me because he threw it back, and luckily I hit the ball pretty good." 
Montero and the Cubs will take on the Dodgers for Game 2 of the NLDS tonight. Tune in at 8 p.m. ET on FS1.