As Texas A&M's offensive coordinator last season, Kliff Kingsbury started Heisman Trophy winner Johnny Manziel at quarterback.
In his first game as the head coach at Texas Tech, Kingsbury will be starting a true freshman walk-on.
Mayfield beat out fellow freshman Davis Webb, who is on scholarship. The two had been splitting time with the first-team offense, until last week when Mayfield began running the majority of the snaps.
Mayfield will face off against SMU quarterback Garrett Gilbert, who starred at the same high school in Austin, Texas, as Mayfield. Out of Lake Travis High, Gilbert originally signed with Texas and won the starting job, before losing it and transferring to SMU.
Like Gilbert, Mayfield put up monster numbers at Lake Travis, throwing for 6,255 yards and 67 touchdowns with a record of 25-2 in his two seasons as the starter. His recruitment, however, never got traction, and Mayfield's only FBS scholarship offers were from Florida Atlantic and New Mexico. Mayfield elected to walk on at Texas Tech instead.
"Baker is a player," Lake Travis coach Hank Carter told ESPN.com last week. "Such a smart and tough competitor. That would be the thing I'd say about Baker. He welcomes the underdog role and he always has."
Texas Tech went into the year expecting Brewer to capture the starting job. But Brewer has been dealing with a stress fracture of a vertebrae since the summer, which the Austin American-Statesman first reported earlier this week.
A source told ESPN.com that Texas Tech is hopeful Brewer might be able to return Oct. 5 against Kansas after an open week.