CHICAGO (CBS)–White Sox fans at Thursday’s game against the Orioles reacted to the suspension of catcher Welington Castillo, who is out for 80 games after testing positive for Erythropoietin, a performance-enhancing drug.

Castillo played his last game before the suspension on Wednesday night, when the Sox beat the Orioles 11-1.

Thursday afternoon’s game against the Orioles played out much differently, with the Sox losing 9-3.

The MLB’s announcement Thursday that Welington tested positive for performance-enhancing substances dampened spirits both on and off the field at Guaranteed Rate. Fans like Steve Cheesman expressed disappointment in the situation.

“It’s an unfortunate thing,” Cheesman said. “He was playing really, really well (but) you never know about these things—if it was something that was a mistake on their part or was it something they were trying to do to get an edge.”

Castillo played in 33 of the team’s 44 games this season. He had been slated to start Thursday’s series finale against Baltimore.

The substance Erythropoietin, or EPO, is used for what is known as “blood doping,” and increases the production of red blood cells, delivering more oxygen to the muscles to increase performance.

Sox fan Gus Kurman said professional athletes should know better than to use illegal substances.

“They make the big money and they’re risking a lot by doing it,” Kurman sad. “They get caught, (so) it’s only fair they get suspended.”

In a statement provided by the Major League Baseball Players Association, Castillo apologized for “an extremely poor decision.”

“The positive test resulted from an extremely poor decision that I, and I alone, made. I take full responsibility for my conduct. I have let many people down, including my family, my teammates, the White Sox organization and its fans, and from my heart, I apologize. Following my suspension, I look forward to rejoining my teammates and doing whatever I can to help the White Sox win.”

Dr. Mark Robert Hutchinson, head team physician at the University of Illinois at Chicago, said the reasons for an athlete to use EPO were unclear.

“It’s a drug doctors use for anemic patients to help them build their blood,” Hutchinson said. “It occurs naturally in the body.”

“If you build up your blood, have more red blood cells, you carry more oxygen, have more endurance,” he continued.

White Sox general manager Rick Hahn said the team was “saddened and disappointed” to learn of Castillo’s suspension:

“Welington has apologized to the team and his teammates and has taken full responsibility for his actions. He understands that he has negatively affected the team and has fallen short of the expectations we have of our players. The White Sox fully support Major League Baseball’s policy and its efforts to eliminate performance enhancing substances from our game.”