KANSAS CITY, MO. - Matt Tolbert started at shortstop for the Twins on Saturday, one day after Alexi Casilla had another poor game.

As the second month of the season has arrived, the Twins could be nearing a decision on the erratic Casilla, who is batting .167, has committed three errors and failed to make other plays, including a key one that resulted in the tying and winning runs scoring for Kansas City in a 4-3 victory Friday.

Will they support him and hope he improves, or will they consider a change at such a key position?

"There have been some pretty good moments when he has gotten on base and has made some nice plays," manager Ron Gardenhire said. "Just like everybody else, the focus gets turned -- when you're not scoring runs -- on a lot of different people. We have to get a lot of people back on the field and the focus would be taken off of a few people."

Tsuyoshi Nishioka continues to make progress in his recovery from a broken left fibula. He has been cleared to do more land-based exercises and baseball activities after running on an underwater treadmill for several days. The Twins still believe he is on a four-to-six week timetable of recovery, making a mid-to-late May return possible.

Asked if Nishioka could move to shortstop, Gardenhire didn't really shoot it down, saying: "Well, just get Nishi well and we will make all the decisions on when and where. We know he can play both sides of the infield. Whatever makes our team better is what we'll do when Nishi comes back."

Nishioka playing short would allow Gardenhire to leave Michael Cuddyer at second base, which in turn would put Jason Kubel in right field and Jim Thome at designated hitter.

Baserunning matters Gardenhire continued discussions about baserunning -- with his players and the media -- Saturday, a day after the Twins committed a few baserunning gaffes during Friday's 4-3 loss to the Royals. Third baseman Danny Valencia failed to score from second on a single and made mistakes on the basepaths in other games this season too.

Preparation is important, Gardenhire said. A baserunner should know where opponents are aligned on the field to help him make snap decisions when trying to take an extra base or scoring a run.

"Right now, [Valencia is] a little nervous about screwing up on the bases, and you can't run the bases like that," Gardenhire said. "We told him, 'You gotta run, son.' "

Regardless if a baserunner has good instincts or not, he should be aware of his surroundings. Gardenhire said those things were discussed with Valencia and others on Saturday.

"You know it right away when a guy has good instincts on the basepaths," he said, "but there's a lot of guys who are not fast who actually pay attention to things and read balls and know when it is hit that this guy is over here and this guy is over there. It's instincts, but is also paying attention."

Mauer update Twins trainer Dave Pruemer said catcher Joe Mauer's soreness is decreasing and that he is making good progress in dealing with his bilateral leg weakness. But he's not ready to increase his activities.

"He wants to see a good strength increase where he feels comfortable to get back to swinging and throwing," Pruemer said of Mauer, who has not played since April 12.

Etc. • Righthander Kevin Slowey (shoulder bursitis) is set to start for Class A Fort Myers on Monday as he nears a return.

• Thome returned to the lineup Saturday after missing four games because of an oblique strain.