Mauer, who has been taking ground balls at first base during batting practice since June 27, hasn't played the position since appearing in one game there with Class A Fort Myers in 2003.
"He's been taking plenty of ground balls, and we want Jim Thome in the lineup over there with [Drew] Butera going to catch [Carl] Pavano," manager Ron Gardenhire said Wednesday. "So we gave Joe a little bit of a break today -- he got one at-bat -- but he can be off his legs again tomorrow and play first base."
Mauer, who struck out as a pinch-hitter with the bases loaded to end Wednesday's 12-5 loss, is being called to action at first with Justin Morneau out until at least mid-August after undergoing surgery last week to remove a herniated disk fragment from his neck. It will be a change of pace for Mauer, who last saw extended time at first when he played 13 games there with Class A Quad Cities in 2002.
Mauer said he'll use one of Michael Cuddyer's first baseman gloves and admitted "he's a little nervous" about his first game at first in the big leagues.
"I got a couple days of ground balls in, and he asked me to go over there and play first, so that's what I'll do," Mauer said. "I haven't played a game yet, so I don't know [what to expect]. You have to get out there and play, and that's where I'll be [Thursday]."
Gardenhire said he likes what he's seen from Mauer so far, but he hasn't had a chance to work on any game scenarios, as Mauer has only taken ground balls and worked on his footwork around the bag.
"He's fine," Gardenhire said. "But you won't really know until you get him into a game. He's pretty fluid over there, to tell you the truth. He has soft hands and moves around pretty good. All the fundamental parts of it will be a learning experience for him."
With Mauer out of the lineup on Wednesday, Rene Rivera started behind the plate and caught Francisco Liriano, who struggled and allowed five runs over just 4 1/3 innings.
Mauer admitted this season "will be a grind," and indicated he's still feeling the effects of the bilateral leg weakness that kept him out of action for two months.
"They feel about ... well, me not playing today probably tells you," Mauer said of his knees. "I want to be in that lineup every day. I understand what it's going to be like the rest of the year."