Joe Mauer rarely admits to frustration, but seven weeks into the season, he has looked less than comfortable at first base, hasn't driven the ball with regularity or authority and is seeing his line drives consumed by opponents' defensive shifts.

"That left fielder is getting pretty aggressive with where he plays me," Mauer said. "That's been tough."

Following the Twins' 6-2 loss to Felix Hernandez and the Mariners on Sunday, the Twins star is hitting .283 with a .373 on-base percentage and a .352 slugging percentage. His career numbers entering this season in those categories: .323, .405 and .468.

This would be a good time to question Mauer's worth, if it weren't for one recent development:

Mauer injured his back. Other than a concussion, there might be no more insidious injury for a professional athlete, especially one expected to play every day, than a bad back. Late Sunday afternoon, Mauer, in his usual understated manner, hinted that the back injury that cost him five games in early May was serious and lingering.

"It grabbed," Mauer said. "Every sneeze, every movement, right away, it grabbed. I'd be trying to get ready for the game, and every time my heel hit the ground, it was shooting pain. I had to calm it down. I was hoping I could get back in the lineup a little sooner, but I was able to get back in there after a couple of days, and I haven't had any problems since."

When Mauer said, "It grabbed," I winced. When my back has "grabbed," I have found myself taking two hours to get out of bed, while screaming. I have been relegated to sleeping on the floor and gobbling painkillers. A severe back injury can prevent you from tying your shoes, not to mention hitting a big-league fastball.

"It's funny," Mauer said. "When it happens, there are so many things that are trying to protect that area. You're working on your back but you're also working on other things. Getting adjustments and treatments. I feel we have a good plan going forward.

"It's baseball, too. You don't have a lot of time to recover. You do a lot of maintenance stuff before and after games, not just the back but the whole package."

Mauer might need a medical liaison.

His reputation among many Twin Cities sports fans was damaged when the phrase "bilateral leg weakness" was attached to his ailments in 2011. He never had anything like "bilateral leg weakness," nor did he ever speak that phrase, but his unwillingness or inability to explain the injury that derailed that season left him looking reluctant.

His latest injury was described as "back spasms." "Spasms" connotes a fluttering muscle. When Mauer said his back "grabbed," he was describing sharp, searing, debilitating pain. Yes, that can be described as a muscle "spasm," but that word does not communicate severity.

"I'm starting to feel better," he said. "I was a little under the weather all week. I'm getting back in the weight room for the first time since it happened, so that felt good."

The Twins hoped that Mauer's transition to first base would make him more durable and productive. Neither has happened yet. He is such a gifted athlete that the Twins expected him to quickly become a Gold Glove-caliber fielder. Perhaps because of the back injury, Mauer doesn't look as smooth as expected.

The Twins led 2-1 Sunday when starter Ricky Nolasco got Mariners speedster James Jones to hit a sharp grounder right at Mauer. Mauer bobbled it, barehanded it, then looked to flip to Nolasco. Nolasco didn't cover the base.

That led to a two-run inning and to Nolasco's early departure, and it might have cost the Twins the game. "Ricky said he was a little slow getting over," Mauer said. "The guy ended up scoring, so that hurt us in the end. He said, 'That was my bad, I should have been over there.' We've got to make that play."

Mauer has not looked, or swung the bat, like himself.

Sometimes the simplest explanation is the most accurate. An injured back is the simplest explanation.

Jim Souhan can be heard weekdays at noon and Sundays from 10 to noon on 1500 ESPN. His Twitter name is @SouhanStrib. • jsouhan@startribune.com