Andrew Butterfield, an art dealer and Renaissance scholar, had seen the two-and-a-half-foot tall wooden sculpture several years before, in a photograph, and thought it was “really fantastic.”

“It felt so much like the embodiment of the early Renaissance,” he said recently. He passed on making an offer then. But the gilded figure of a plump, graceful cherub, or putto, nagged at him, and when he finally did buy it, in 2012, it set him off down an art-historical detective trail that made him glad he followed his instincts. Mr. Butterfield and several other experts he has enlisted now believe the statue is a lost work by Donatello, one of the defining artists of the Renaissance, and a rare example of the artist’s work in wood, making the discovery not only a major addition to Donatello’s surviving corpus but also to the history of Western sculpture.