Purdy (25) was a limited participant in practice on Friday and will be used as an emergency backup if he's needed on Sunday, stating, "I'm trying to be smart with my body."
Jones started in the team's Week 2 win over the New Orleans Saints, completing 26-of-39 passes for 279 yards and three touchdowns, including six rushes for six yards.
It'll be Jones' ninth start dating back to last season when he was on the Jacksonville Jaguars.
San Fran and Arizona both are heading into the matchup with a 2-0 record as a crucial NFC West showdown that could possibly have potential benefits and repercussions, depending on who wins and loses, for either side towards the end of the season.
The 49ers are 2.5-point favourites over the Cardinals with the over-under set at 44.5.