And on the second pitch that Decker saw on Friday against the Giants at Scottsdale Stadium, Decker crushed a home run to almost straightaway center.
On Saturday morning, Decker wasn't gloating, but he smiled and told the reporter, "I told you."
Actually, Decker has gotten the attention of manager Ned Yost and the coaching staff.
After signing a Minor League deal in the offseason, Decker, 29, is still in camp, still on Yost's radar.
"He's done a great job at first [base]," Yost said. "He has swung the bat very well for us."
And that is Decker's strength -- his bat. He finally got a cup of coffee with the Padres last season after seven somewhat prolific seasons in the Minors.
Decker has hit 154 homers with a .871 OPS over his Minor League career.
Decker's value to the Royals is his versatility, and he's in a battle with veterans Clint Barmes and Travis Snider, and youngsters Terrance Gore and Whit Merrifield, for possibly the 25th spot on the roster.
"Utility guy," Yost said. "He came in early with the catchers and caught. He could be a big-time emergency catcher. He's very proficient. He can play third. He is not as polished at all positions as Merrifield is, but he can play all positions."