The Twins placed Mauer on the disabled list on April 14 with bilateral leg weakness. Smith said on Tuesday that Mauer lost about a week of rehab time because of a nasty bout of the flu that circulated through the clubhouse.
Smith says Mauer has since recovered from the illness, but still needs more time to strengthen his legs for the rigors of catching. The team has not determined if Mauer will need to go to the minors on a rehab assignment before he joins the Twins.
Mauer is hitting .235 in the first season of an eight-year, $184 million contract extension. He had arthroscopic surgery on his left knee in the offseason and only played sparingly in spring training to try to conserve himself for the regular season.
Smith said infielder Tsuyoshi Nishioka "continues to make good progress" while rehabbing his broken left leg. He's done some throwing and continues do leg work in the pool, but is still "weeks away."
Right-hander Kevin Slowey, on the disabled list since April 9 because of bursitis in his throwing shoulder, will throw about 45 pitches in extended spring training on Wednesday.