"We've got to keep getting him out there on somewhat of a consistent basis if we want to get him better," Hurdle said. "He's not going to get better not pitching."
Trying to protect a one-run lead in the seventh in his first true pressure situation since returning from the disabled list last week, Grilli (0-2) surrendered a towering two-run homer to Anthony Rizzo that ended up snapping Pittsburgh's four-game winning streak.
Grilli struck out the first two batters he faced in the seventh before walking Luis Valbuena on a borderline 3-2 pitch. Grilli hopped in frustration after the pitch was ruled a ball. All he could do was look down in disgust when Rizzo broke out of a long slump and sent an 83 mph slider over the stands in right to put Chicago in front to stay.
"He threw me a slider down and in, a pretty good pitch, and I tried to just put a good swing on the ball and not try to do too much," Rizzo said.
Brian Bogusevic went 3 for 4 with his fifth homer of the season for Chicago. Dioner Navarro added two hits.
Carlos Villanueva (6-8) picked up the win in relief. Kevin Gregg earned his 32nd save when Neal Walker's shot to center with one on in the ninth died at the warning track.
Pedro Alvarez, Russell Martin and Garrett Jones hit consecutive solo home runs in the fourth for Pittsburgh. Andrew McCutchen went 2 for 4, but the Pirates lost for just the fourth time in 65 games when leading after six innings this season after Grilli faltered.
Grilli, who made the first All-Star team of his 11-year career after racking up 29 saves in the first half of the season, is still working his way back into form. Hurdle moved the 36-year-old into a middle relief role when he was activated after Mark Melancon proved nearly automatic in the closer's spot in Grilli's absence.
It's unlikely Grilli will get his old job back anytime soon, though don't expect Hurdle to hide Grilli on the bench down the stretch.
"There's no safety net at this level and we believe in him and we're going to keep giving him the ball when it makes sense and we're not going to let him sit too long," Hurdle said. "We'll try to find the best spot available."
Chicago's bullpen made Rizzo's blast stand up, silencing Pittsburgh over the final three innings after the Pirates erased an early 3-0 deficit against starter Jake Arrieta.
Arrieta breezed through the first three innings and was on his way to doing it in the fourth when Pittsburgh suddenly sprung to life.
Alvarez hit a fly ball to the wall in right-center with two outs that neither Sweeney nor right fielder Nate Schierholtz could corral. The 235-pound Alvarez chugged all the way around the bases for his 33rd — and by far most taxing — home run of the season.
The Pittsburgh third baseman was still catching his breath when Martin followed with a shot 10 rows deep into the stands in left to bring the Pirates within one. Jones tied it seconds later by sending a laser into the seats in center. It marked the first back-to-back-to-back home runs by the Pirates since Jason Kendall, Brian Giles and Reggie Sanders did it on Aug. 20, 2003, in St. Louis.
Pittsburgh eventually took the lead in the sixth when reliever Brooks Raley mishandled a lob from Rizzo at first base, allowing McCutchen to race all the way home from second.
The Pirates have risen to the top of the NL Central behind the strength of a bullpen led by the irrepressible Grilli. His velocity, however, has dipped since his comeback and Rizzo took advantage.
NOTES: The Pirates will consult with specialist Dr. James Andrews before figuring out the next step for LHP Wandy Rodriguez, who has been out since June with left forearm tightness. Rodriguez threw 67 pitches in a simulated game on Thursday ... The series continues on Saturday when Pittsburgh's Gerrit Cole (7-7, 3.48 ERA) faces Chicago's Scott Baker (0-0, 0.00). Baker is making his second start since reconstructive elbow surgery.