Stewart had left the clubhouse by the time it was opened to the media following the cancellation of Monday's game against the Yankees at McKechnie Field. Manager Clint Hurdle didn't know if there was one particular play on which Stewart sustained the injury, nor did he have an official diagnosis or estimate for how long his catcher will be out.
"I wasn't even really aware of the fact until we asked if he wanted to stretch out for one more at-bat [in the game against Minnesota]," Hurdle said. "He said, 'No, I'm good.' [His knee] was a little tight."
The Pirates acquired Stewart, 32, in a trade with the Yankees in December and planned to have him back up Russell Martin. Stewart has hit just .214/.287/.288 over parts of seven Major League seasons with the White Sox, Rangers, Yankees, Padres and Giants but is well regarded for his defensive ability.
Stewart's injury likely opens the door for 25-year-old Tony Sanchez to crack the Opening Day roster. Sanchez made his Major League debut last season, hitting .233 with two homers and five RBIs in 22 games. Sanchez has hit well this spring, posting a .296/.345/.556 line with two homers and six RBIs in 12 games.
The Pirates are comfortable with the idea of entering the season with Sanchez as their backup catcher, Hurdle said, based on his experience down the stretch last season.
But Pittsburgh has little depth beyond Sanchez, with Nevin Ashley and Omir Santos the only options in Major League camp. Both players were invited to big league Spring Training after signing Minor League contracts. That concern could lead the Pirates to seek another catcher through the trade market rather than promote Sanchez.
"Basically, it's the same question that we had before we acquired Chris: What's the best scenario for Tony's development, long range versus short range?" Hurdle said. "If it's a short-range [injury] for Chris, is Tony the guy? If it's longer-range -- those are the discussions we'll have based on when we get more information."
Adam Berry is a reporter for MLB.com. Follow him on Twitter at @adamdberry. This story was not subject to the approval of Major League Baseball or its clubs.