Hartnett has resumed his role in London's West End at the Apollo Theatre as the younger brother scheming to use his autistic-savant older brother's math wizardry to save his business.

Since November 17, according to Broadway World, the last four performances of the production were Josh-free.  The leading man had been briefly hospitalized due to intense stomach pains.

Speaking of causing intense pain, Harnett was publicly linked with celebrity husband snatcher Sienna Miller, 26.

Miller, who recently ended her brazen adulterous tryst with 33-year-old married actor Balthazar Getty, was seen with Hartnett at Shoreditch House in London prior to his mystery illness.

“They looked like a couple—and a good looking one at that!” a source said. “There was definitely a buzz in the air.”

Careful, Josh. Sienna's a wild child, and you've only begun to heal from that one dinner.