The tumor was discovered last week during a colonoscopy.
Lab results and the timetable for Mancini's recovery will not be known until next week.
"The outpouring of love and support I have received has made an extremely tough week so much better," Mancini said in a statement. "I have the best family, friends, fans, and teammates imaginable. I am also eternally thankful for the Orioles front office, our athletic trainers, and the entire medical staff for everything they have done to help me during this time. Finally, I would like to thank everyone for their prayers and kind words, which have furthered my excitement to get back to playing the game I love."
On Saturday, the Orioles announced Mancini had left the team to undergo what it called a "non-baseball medical procedure."
"We are doing everything in our power to ensure Trey recovers fully, and we can't wait to see him back on the field as soon as possible," said Mike Elias, the team's executive vice president and general manager.
The 27-year-old Mancini, who also saw time at first base and designated hitter last season, hit .291 with 35 home runs and 97 RBIs. He played only five games this spring after earlier dealing with flu-like symptoms.
Mancini, who will turn 28 on Wednesday, last played on March 2, but he left the spring training game after two at-bats.
Mancini was one of few bright spots for the rebuilding Orioles last season. In addition to playing in 154 games and leading the team in homers and RBIs, Mancini played the role of clubhouse leader on a team filled with youthful players.
Though Baltimore has lost more than 100 games in each of the past two years, Mancini has been a standout offensively. In 2018, his second full season, he hit 24 home runs over 156 games.
Mancini made $575,500 last year. After reaching career highs in homers and RBIs, he was given a raise to $4.75 million in 2020.
Information from The Associated Press was used in this report.