Dunn said the injury occurred when he dived into third base in a game against Houston in May. Later in the season, he said he was hit by a pitch in the same spot.
It wasn't clear if former Reds manager Dave Miley or the Reds front office were aware of Dunn's injury.
Dunn said his hand still hurts but he downplayed the injury. It was checked this week, and the doctor recommended he put a splint on it, he said.
"The heck with that," Dunn said.
Dunn is expected to move from the outfield to first base this season. He hit .247 with 40 home runs and 101 RBI last season.