"There's been a lot of pain, probably the most pain I ever felt," Tucker said.
The Nationals placed Tucker on the injury list with a strained right groin, an injury suffered in a 6-3 loss to Florida Tuesday night.
Tucker said he hurt himself while throwing a pitch to Juan Pierre in the sixth inning.
"I don't know what happened," Tucker said. "It just popped."
The right-hander was helped off the field and later used crutches as he left the locker room.
Tucker will be out of the lineup 2-4 weeks, team officials said.
"Tucker's not good," Manager Frank Robinson said. "It's going to be a big loss. It's going to be a big hole in the bullpen for us."
Tucker (1-0) has allowed four hits and no runs in 5 1/3 innings of relief this season.
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend
Google buys drone maker Titan Aerospace