Burnett struggled through his last start, against the Astros, but that's nothing compared to what Lincecum has been through this season. The once-dominant righty has a 6.08 ERA and has been wildly inconsistent all season. His velocity is way off this year (he's lost about two MPH on his fastball compared to last season), and he's having lots of trouble with runs on base, leading to big innings.
Lincecum's last outing, in which he allowed seven earned runs in 3.1 innings against the Washington Nationals, was awful, but the Giants are (understandably, given his history) sticking by him, starting him not only against the Bucs on Sunday but in the Giants' second game after the All-Star break.
After beating the Giants Saturday, the Pirates have a one-game lead on the Cincinnati Reds.