MLB Managers

Today's Pirate game has me fired up. Why do managers refuse to use their best relievers if it isn't a perfect scenario/ Case in point. Today, the Pirates needed to preserve a lead in the 9th. Who pitches? Tony Watson and Vin Mazzaro. Why? Why wasn't it Melancon or Grilli? Those guys have been perfect. Just because it isn't a save situation, it doesn't mean you cant use your BEST relief pitcher. That just doesn't make any sense. And the funny thing is, it's not just Hurdle. It's ALL the managers. ****ing Stupid.