The team initially said he was taken to a hospital for precautionary reasons before clarifying that team doctors were handling his care. Manager Bruce Bochy said Cueto was being treated for a contusion and monitored for concussion symptoms.
"We are going to monitor him tonight, tomorrow for concussion symptoms, but he's shown no sign of that, so he's just being treated for a contusion," Bochy said after the game. "He responded well out there. It's why he stayed in the game; he answered all the questions from our trainer out there. ... He wanted to stay out there. We did take him back to have our docs look at him at our complex."
Cueto was struck by a ball hit by Oakland Athletics leadoff man Billy Burns. He underwent a quick check of his ability to continue, then completed the inning.
"First night game, first pitch of the game, I'm sure he wasn't recognizing it off the bat," Bochy said. "That's a scary moment. I'm just hoping it's what it was, just a glancing blow, but still, it caught him pretty good."
Cueto surrendered a three-run home run to right field by the A's Josh Reddick following the incident and left the game after three innings. He gave up five hits, three runs and one walk with two strikeouts.
"It showed how tough he is," Bochy said. "His second and third innings were good innings."
The Giants went on to lose 10-2 to the A's.
Cueto's scare came one day after the Giants announced that staff ace Madison Bumgarner would miss a start or two for rib cage "discomfort" and a neuroma in his left foot.
Monday's game marked Cueto's second start of the spring, coming on the heels of his getting ripped for five runs in 1 innings in his Cactus League debut March 9 against the Colorado Rockies.
Cueto came to the Giants from the world champion Kansas City Royals as a free agent in December, signing a deal for at least six years and $130 million. The Giants, looking to shore up their staff, also signed right-hander Jeff Samardzija to a five-year, $90 million deal this offseason.
The Giants have had a number of setbacks since the start of spring training. After two injury-wracked seasons, former staff stalwart Matt Cain is questionable for the start of the season after having a cyst removed from his right (throwing) arm.
Bumgarner is expected to be fine for Opening Day.
If needed, the Giants could turn to righty sinkerballer Chris Heston, who isn't guaranteed a spot in the rotation despite no-hitting the Mets last June.
Cueto hit in head by line drive, stays in game
During the bottom of the first inning, Giants pitcher Johnny Cueto is struck in the head by a line drive off the bat of Athletics center fielder Billy Burns. After being evaluated, Cueto remains in the game.