He stayed mostly healthy for the Giants last season, appearing in 146 games and hitting.288 with 14 home runs and 80 RBI.
Posey was doing well after being struck by a 94 miles per hour fastball in the first inning Monday, a scary moment in San Francisco's 4-1 victory over the Arizona Diamondbacks during its home opener.
"We just couldn't get the big hit", Giants manager Bruce Bochy said after his team's sixth loss in nine games.
He was placed on the disabled list because he experienced concussion symptoms.
Posey dropped into a sitting position after being hit in the bottom of the first, and stayed down while trainer Dave Groeschner sprinted to him, but left the field on his own power.
The trigger in all this was Walker's first-inning pitch that struck the right-handed Posey in the temple of the head. Walker tried to snap his tag but neglected to catch the ball first.
Coming off a 2-5 trip to start the season, the San Francisco Giants were looking at Monday's home opener as a chance to reboot and change direction. It's a good idea, especially for a catcher who faces a lot of wear and tear and opportunity for additional head injuries, to take time before allowing his return.
While San Francisco has welcomed back players who were injured late last season such as third baseman Eduardo Nunez and second baseman Joe Panik, the Giants are still waiting for this group to click.
"It's a scary moment, unsafe moment", Bochy said. So Panik scored, and Jarrett Parker dashed home as well. "It's just three wins we have, I know, but heading forward, I know what we've got here". The lone Diamondback run was a home run off the bat of Yasmany Tomas. Tampa Bay's Logan Morrison homered with one out in the eighth, a ball that hit off the top of the wall in right-center and deflected off a fan before bouncing back onto the field. This past Cactus League schedule, Federowicz hit.323/.417/.645 with seven doubles and a home run in 21 games.
Cain (1-0) overcame a shaky start after allowing a run in the first inning and retired 10 of 11 during one stretch.
Posey was certain to undergo the concussion protocol, and it's unclear how long he might be sidelined.
The series win leaves the Giants at 4-6 entering their first meeting with the Colorado Rockies on Thursday at China Basin. In 2015, he held a 2.85 ERA, earning him Pacific Coast League Pitcher of the Year honors.