DETROIT -- Miguel Sano is one of the best power hitting prospects the Twins have had, but he's really struggling defensively at third base this year in the Class A Midwest League.

Sano, who is batting .244 with 18 homers and 57 RBI, made three errors Monday night for Beloit, giving him 31 for the season. That's 31 errors in 78 games at third base.

"We're always concerned about defense," Twins minor league director Jim Rantz said. "Obviously, the kid is just 19 years old. They're not just throwing or fielding errors, they're both. He's just got to continue to work hard and take ground balls. He's at the stage of his career where he's figuring it out that there's more to the game than hitting; there's fielding, too.

"I know he's concerned, too. The thing is, he'll make the errors, and then he'll turn around and make the good plays, too. So I'm sure he'll stay with it and keep working on it."

History is filled with examples of players who had huge error totals in the low minors but improved enough to thrive in the majors. Michael Cuddyer had 61 errors at shortstop for Class A Fort Wayne in 1998, although given his eventual move to right field, I'm not sure that's the best example. As Seth Stohs, from Twins Daily notes, Derek Jeter made 56 errors in the Class A South Atlantic League in 1993. Jeter's had an OK career.

General Manager Terry Ryan has said there is no reason Sano shouldn't stick at third base. The Dominican phenom has the defensive tools to do so, with a good arm and decent hands.

After making those three errors and hitting his 17th double, Sano left Monday's game with leg stiffness. Rantz doesn't believe the injury is too serious. Sano has a .351 on-base percentage and .509 slugging percentage after 338 plate appearances.

* The Twins have signed Lewis Thorpe, a 16-year-old lefthanded pitcher from Australia, for $500,000. Added to the $1.4 million they spent Monday on Dominican shortstop Amaurys Minier, that would bringing their international signing total to $1.9 million, leaving $1 million under this year's cap.