McElroy, a second-year pro who has thrown only seven passes in his NFL career, has also jumped ahead of Sanchez's back-up Tim Tebow and will start against San Diego Chargers in New York on Sunday.
"After thinking about it, about where the team is now, I think it's best for our team and for this game," said Jets head coach Rex Ryan. "That's what I feel in my gut."
Sanchez, who led the Jets to the AFC title game in each of his first two seasons and still has four years remaining on a lucrative contract, paid the price for an error-riddled campaign.
The final straw came with four interceptions in a Monday night loss to the Tennessee Titans in week 15, a defeat that dropped the Jets to 6-8 for the season and out of contention for a play-off spot.