Mauer is scheduled to undergo a magnetic resonance arthrogram on Wednesday since he has continued to experience discomfort in his lower back nearly 2 1/2 months after undergoing surgery to eliminate a kidney obstruction.

The MR arthrogram will inject dye into Mauer's back and abdomen to get a clear image of the area. It's the latest attempt by the Twins doctors to determine what is still causing the continued discomfort.

"We haven't been able to get him on the field, so to say we're not concerned would be silly," Gardenhire said after the Twins' 4-3 victory over the Rays on Monday afternoon.

"Yes, we are concerned. We've got to figure out what is going on so that we can get him better. And this is another effort to try to figure out where we are at and what we need to do."

The belief by both the Twins and Mauer had been that the catcher would be very close to getting back on the field by this point. Mauer, 25, has been doing light agility drills and conditioning since reporting for Spring Training, but the catcher suffered a setback recently when he tried to do light baseball activities.

With less than four weeks remaining until Opening Day, Mauer has yet to run at full speed, catch anything more than a bullpen or take swings off live pitching. And there is no clear timetable right now as to when the All-Star catcher might do any of that.

The Twins open the season on April 6 against the Mariners at the Metrodome. Since Mauer's surgery, the club has set that as the target date for the catcher to be back to 100 percent.

But with the latest news about Mauer, there seems to be growing concern over whether he might actually be ready at that time.

"He's not on the field yet. He's not able to consistently go out and do anything," Gardenhire said. "So yeah, concern, that's a good way to put it."

The news of that Mauer will undergo the MR arthrogram comes one day after Gardenhire first acknowledged that the catcher's progress has been slower than anyone expected.

On Sunday, Gardenhire said he was still hopeful that Mauer will be ready to open the season with the club, but the skipper acknowledged he was starting to think of alternate plans should that not be the case.

"I'm still holding my ground here, waiting," Gardenhire said of Mauer. "Have I got secondary plans that I'm working on? I'm playing a lot of catchers. If it were to get to a point where he's not going to be able to start with us, then we'll start really locking down on things, but right now, I'm just kind of rolling with it."