In November, it was announced that Grandal would miss the first 50 games after he tested positive for elevated levels of testosterone.
Grandal had been playing with Tucson since May 18, as he was allowed to play in 10 games at the Minor League level before his suspension was up. He hit .306 in nine games with three doubles and two RBIs in 36 at-bats.
Grandal flew to Seattle on Monday and will be on the 25-man active roster when the Padres play the second of two games against the Mariners.
"He's ready," said Padres manager Bud Black.
Baker was hitting .150 in 40 at-bats. He said he wasn't totally surprised by the news, yet he was disappointed.
"The writing was kind of on the wall," Baker said. "It will be nice to be able to play. I'm disappointed in how I swung the bat when I was here. But it's an opportunity for me to get some at-bats."
Other than a Minor League rehabilitation assignment in Triple-A in 2011 when he was with the Marlins, Baker hasn't played in the Minors since '08.
"I'm happy for the opportunity to try and find myself," Baker said.
It's unclear how the Padres will use Grandal. Catcher Nick Hundley, who was 1-for-3 on Monday, is hitting .252 this season but only .143 (8-for-56) in May.
Last season, Grandal ran away with the starting catching duties after he was promoted for a second time on June 30. In his first start, he hit home runs from each side of the plate against the Rockies. He had a .297/.394/.469 line with eight home runs and 36 RBIs in 192 at-bats with the Padres.
"The Padres have given me great backup. My teammates have been great. I've got nothing to say about that. They've backed me up 100 percent," Grandal said last week, while Tucson was playing in Omaha.
Grandal worked out with the team in Spring Training and played in nine games. He remained behind when the Padres broke camp in late March and played with the club's extended spring training team until he was able to join Tucson for a period not to surpass 10 days.