Percival will visit a doctor in Texas on Friday, but the outlook does not look good, as he wore a brace and had a set of crutches in his locker after Thursday's 7-6 win.
With Percival likely headed to the disabled list, the Rays plan to fly a relief pitcher, who would be activated if the prognosis is bad, to Texas. Likely candidates include left-hander Kurt Birkins and right-hander Juan Salas from Triple-A Durham, both of whom are on the 40-man roster. Bulls right-hander Scott Dohman could also be a possibility, but a player would have to be taken off the 40-man in order to make a spot for him.
Percival's injury occurred in the bottom of the ninth. Jack Hannahan drew a leadoff walk against Percival before moving to second on a sacrifice bunt by Mark Ellis. Percival fielded the bunt and chased down Ellis, but injured himself in doing so and had to leave the game.
"I made the play -- I never felt anything pop," Percival said. "It's not the hamstring; it's my knee. When I was walking back to the mound, I felt some sharp pain going on in there and I couldn't put weight on it. It's a sprain of some sort. I don't know."
Percival already has had two stints on the disabled list this season due to a left hamstring problem.
"I'd like to give you a medical opinion," Percival said. "But I have no idea."
Pressed further for a gut prognosis, Percival replied: "I don't even like to give those any more, because my opinion is I'll be back out there in two days. And most of the time, it doesn't work out that way."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.