Harris worked with the starters the entire preseason as Gaither hasn't practiced since his back spasms developed. The Chargers aren't sure when he'll be able to return.
"I don't know if there's been a setback," Chargers coach Norv Turner said Saturday, according to U-T San Diego. "He hasn't made any progress. That's for sure."
Gaither, who helped solidify the Chargers' line last season during the last five games after he was waived by Kansas City, signed a four-year contract with San Diego in the opening days of free agency.
"It's a very disappointing situation we have been dealing with the entire training camp. Jared is hurt and not available to us. The clock struck midnight for him after our last preseason game, and we are moving on with Mike Harris," general manager A.J. Smith said Saturday, according to U-T San Diego.
"Mike will get the first opportunity to hold down our left tackle position. He is a talented kid with a bunch of ability. He is excited about this opportunity, and we are all excited for him."
The 6-foot-5, 310-pound Harris called the opportunity to start a "dream come true."
"The opportunity was given to me when tackles went down, and I was able to overcome. I fought my way up from third string to second string to first string, and I've been able to hold that spot down. That's what I plan on doing, staying in there," he said, according to the North County Times.