Cuellar, a four-time All-Star, helped pitch the Orioles into the World Series in three consecutive years (1969-71). He closed out the 1970 championship with a win over Cincinnati in Game 5.
On Saturday morning, Baltimore manager Dave Trembley broke the news to his team in what he called one of the toughest meetings he's ever had.
"If there was ever a guy who epitomized what the Orioles are all about, it's Mike Cuellar," Trembley said. "All he ever wanted to do was help [the organization]; it's really a big loss."
Trembley said he and Cuellar would talk on the phone once a week during the offseason, but Baltimore's manager had lost contact with Cuellar in the weeks leading up to Spring Training.
"His phone wasn't accepting any calls. His cell phone was off or you'd get a recording," Trembley said. "Then about seven or 10 days ago, [Hall of Famer Jim] Palmer told me when we were in Jupiter. Palmer said, 'Hey, he's not going to make it.'
"That's really been on my mind a lot. Just terrible. But we've got to remember him for all the good things he did."
Known for his sharp screwball, Cuellar is a member of the Orioles Hall of Fame, and in 1969, he became the first Baltimore pitcher to win the American League Cy Young Award, sharing the honor with Detroit's Denny McLain. The left-hander won 143 games with the O's and lost 88. In franchise history, he ranks second in complete games (133) and third in shutouts (30).
His last victory for Baltimore came in 1976, when he was 39.
"To watch him pitch was amazing," Orioles reliever Dick Hall told the Baltimore Sun. "It seemed like every time hitters took a pitch, it was right at the knees for a strike, and if they swung, it wasn't."
Overall, Cuellar went 185-130 with a 3.14 ERA for his career, which began in 1959 with Cincinnati and ended in 1977 with the Angels.
"Mike was a monstrous part of the great teams we had from 1969 to 1971," Hall of Fame manager Earl Weaver told the paper. "He was an artist on the mound and a player [whose acquisition] put us over the top. Several times, down the stretch, he pitched with two days' rest, when we needed it."
"I certainly hope his place in Orioles history will be remembered in a way that's deserving," Trembley said. "[He was] just a great guy."
Brittany Ghiroli is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.