Manager Fredi Gonzalez said McCann's condition is worse than it had been the previous two days and that reserve Matt Diaz is also feeling the effects.
McCann went 0-for-3 in Monday's opener against the Reds, but has been replaced by David Ross in the lineup since.
"I think it's a full-blown flu going now with his temperature and aching feelings," Gonzalez said. "The last three days he's been curled up on a sofa or the training room. There's nothing you can do but let that thing take its course."