"The A's treated me amazingly during all this time, but sadly didn't have space for me and this is something I can't control," Ramirez told ESPNDeportes.com's Enrique Rojas by phone. "I'm going home to continue my training hoping to get the chance to play again. I'll be waiting for that call and if God believes (playing) is the best for me, then it'll happen. If not, I'll understand.
"It was an amazing experience. I was given tremendous treatment by Oakland's major league and Triple-A staff. I'll always have them in my heart. I was ready to play, but Oakland didn't have a space in its roster. I understand."
Ramirez signed a minor league deal with Oakland on Feb. 20, and hit .302 with 14 RBIs in 17 games with Triple-A Sacramento. He served a 50-game suspension for violating Major League Baseball's drug policy last year, but remained in the minors when the ban ended on his 40th birthday on May 30.
The dreadlocked slugger was expected to make approximately $500,000 if he was added to Oakland's big league roster.
"Manny believes he has demonstrated that he is ready to return to the major leagues," Ramirez's agents, Barry Praver and Scott Shapiro, said in a statement. "However, given that the Oakland Athletics could not give Manny any assurance that they plan to promote him in the immediate future he asked for his release. Manny thanks the A's for providing him with this opportunity."
Ramirez, who has 555 career homers and 1,831 RBIs, retired last season rather than serve a 100-game suspension for failing a second drug test. But commissioner Bud Selig cut the suspension in half after Ramirez sat out the balance of the season.
He served a 50-game suspension in 2009 while with the Los Angeles Dodgers' organization, playing parts of three games in Albuquerque during his rehabilitation assignment.
The 12-time All-Star went 1 for 17 (.059) in five games last season for Tampa Bay, which had signed him to a one-year deal worth $2.02 million.
Information from ESPNDeportes.com's Enrique Rojas and The Associated Press was used in this report.