The outfielder is headed to join the Tampa Bay Rays as part of the deal, a year after establishing himself as one of the National League's top outfielders -- and providing yet another shake-up for the struggling Cardinals in the process.
"I was really shocked," Pham said in an interview on MLB Network Radio. "When you wake up to your boss calling you and telling you you got traded, it's shocking."
St. Louis sent the 30-year-old and $500,000 in international signing bonus allocation to the Rays for a trio of minor leaguers: outfielder Justin Williams, left-hander Genesis Cabrera and right-hander Roel Ramirez.
The move is another dramatic change for a disappointing St. Louis team that entered the day fourth in the NL Central and fired former manager Mike Matheny in mid-July. The Cardinals are likely to miss the postseason for a third straight year for the first time since 1997-99.
"It hurts, specifically for me because I've been underperforming from my expectations," Pham said on MLB Network Radio. "I feel like if I had just done my job better, we wouldn't be so far down in the standings."
Pham has been a member of the Cardinals organization since selected on the 16th round of the 2006 amateur draft. After a series of injuries, the center fielder had a breakout season in 2017 when he hit .306 with 23 homers and 25 steals in 128 games. He was 11th in the NL MVP voting.
Pham is hitting .248 with 14 home runs, 41 RBI and 10 stolen bases this season. He talked during spring training of a 30-30 season in homers and steals, and he batted .358 in April before dropping to .197 in May and June.
Pham is eligible for salary arbitration for the first time next winter, and Rays manager Kevin Cash said he would start Wednesday night against the Los Angeles Angels.
"This is a total player," Tampa Bay general manager Erik Neander said. "Yes, he's 30, yes, we're talking about a young core of players, but it's three years of control with a guy we think has the potential to be an impact outfielder."
His departure could lead to more playing time for Harrison Bader, who is hitting .265. The Cardinals also called up outfielder Tyler O'Neill, who is hitting .311 with 26 home runs in 61 games at Triple-A Memphis, and promptly put him in the lineup against the Colorado Rockies on Tuesday night.
"It's not about me and what this creates for me as a player individually," Bader said. "This is about how this opening will help this team win and make a push."
Williams, 22, is a former second-round draft pick and left-handed-hitting outfielder who was batting .258 for Triple-A Durham. He made his major league debut this month and was an International League All-Star.
Cabrera, 21, is 7-6 with a 4.12 ERA in 21 games for Montgomery of the Double-A Southern League. Ramirez, 23, was 3-1 with a 3.32 ERA in 26 games as a reliever for Montgomery.
Also on Tuesday, the Cardinals traded minor league outfielder Oscar Mercado to the Cleveland Indians for minor league outfielders Conner Capel and Jhon Torres.
It was the trading of Pham, however, that had Mozeliak answering questions about another shake-up to a Cardinals team that's failed to meet expectations this season.
Mozeliak said the trade had nothing to do with an article in Sports Illustrated earlier this season in which Pham complained about not being called up from the minor leagues sooner.
"He was in the organization for 12 years, injured a lot, a lot of ups and downs, but in the end we were the ones that believed in him," Mozeliak said. "Most people who have his track record don't make it here."