Right-handed reliever Billy Sadler was recalled from the Grizzlies and watched the 3-1 loss to the Pittsburgh Pirates from the bullpen.
Hennessey posted a 12.94 ERA in 11 appearances with the Giants and had one of his worst outings Tuesday night, allowing five runs and seven hits in 2-1/3 innings in relief of Jonathan Sanchez in a 12-6 blowout.
Manager Bruce Bochy said Hennessey will be a starter with the Grizzlies.
"The best way to get back on track is to go out there and get some consistent work," Bochy said.
The former first-round pick had a breakout 2007 when he had a 3.42 ERA in 69 appearances and was expected to be a key piece to the Giants bullpen. He appeared in four of the Giants' first eight games but allowed eight runs in five innings and fell to the back of the bullpen. Tuesday was his third appearance since April 18.
"It's just a matter of work," Bochy said. "He agreed. It's hard to get that done up here when you're not out there on a consistent basis."
Sadler's arrival bolsters a Giants bullpen that has been taxed on this trip. Relievers have absorbed 18-1/3 of the 44ª innings Giants pitchers have thrown on this trip. Only two starters have pitched six innings in five starts.
Sadler made his big league debut late in the 2006 season.
Not much run support
As has been the case most of the season when Barry Zito starts, the Giants couldn't muster enough offense to get him off the hook.
The Giants had three of their seven hits in the ninth inning on Wednesday and were three outs away from being shut out for the third time in Zito's seven starts. It was the third time the Giants have scored one run in a game Zito has started.
Injuries in the infield
The Giants got a look at their likely middle infield of the future: Brian Bocock at shortstop and Emmanuel Burriss at second base. Burriss, who had played only shortstop since he was called up on April 20, started at second base so Ray Durham (hamstring) and Eugenio Velez (left hand) could rest nagging injuries.
Durham's only start on the trip was in the opener Friday in Philadelphia. Bochy said it will be determined Friday if Durham will be placed on the disabled list.
Stormy weather ahead?
Severe weather forecast for the region could threaten today's series finale, but Bochy said he will not alter the rotation if the game is postponed. Right-hander Matt Cain is the Giants' next scheduled starter.
-- Laurence Miedema, MediaNews staff