I wouldn't drop Eli for Garrard but if I had another way of picking up Garrard I'd get him and start him vs the banged up Seahawk defense. NYG say that Eli will not play if he is limited.
From cbssports.com,
While his painful heel injury is improving, Eli Manning is not guaranteeing he will quarterback the Giants this weekend against the Raiders. Manning, who has started the last 82 games, was held out of practice Wednesday so he can get treatment on his plantar fascia, a band of connective tissue that runs along the bottom of the foot and inserts into the heel. Manning admitted his heel bothered him occasionally the last few weeks. However, he never had pain like he felt in the fourth quarter Sunday when he set up to throw against Kansas City and literally had to hop because it so was intense. He has asked the team's medical staff whether playing this weekend might make his injury worse. They told him they just don't know. "As an athlete you know when something is bothering you and when something is going to get worse or not, and I'll make that decision," Manning said.