"It's possible it is worse than that one, so we have to wait and see," Matheny said.
Matheny said the organization will wait until Monday before having Schumaker take an MRI on the strained muscle area.
"It's not the type of injury where we have to rush him into one, because we already know what he's dealing with," Matheny said. "The biggest thing for him to do now is rest and recover."
Schumaker left Friday's game early after sustaining the injury during his fourth-inning at-bat. He had little to say about the injury postgame, but he did characterize it as worse than the strain he endured last fall.
That injury kept Schumaker off the NL Championship Series roster. He did return to play in the World Series.
Matheny said reading players and their tendencies is one of the things he is learning as a first-year manager. On Friday, he voiced disappointment about not stepping in to perhaps prevent the injury. One pitch before Schumaker felt his side really tighten, Matheny noticed Schumaker grab at the area.
"I asked if he was all right, and he told me that he was," Matheny said. "My gut told me that he wasn't, and [I] let him [keep hitting]. That made it worse. Neither of us did the right thing."
Schumaker, who was the designated hitter in the game, ended up grounding out. He was pinch-hit for in his next at-bat.
"I'm still learning all their idiosyncrasies up close," Matheny said. "I've watched them all, obviously, for a while. But there are guys that [wince] every time they take a swing. But that one, I saw it, and I didn't follow through."
Matheny said oblique injuries are difficult to place an exact timeline for recovery on, so it is "tough to determine when he might return" to any type of activity, let alone a game.
Cardinals pitcher Lance Lynn missed nearly two months with an oblique strain last season. Matheny noted that he was out a month when he sustained one during his playing career.
Schumaker was in the middle of a three-way competition for the starting second-base job. He had also been getting regular work in the outfield in case he ended up in a utility bench role.