Ehlinger had an impressive debut in the Longhorns' 56-0 rout, going 15-for-27 passing for 222 yards and a touchdown pass.
Buechele, who is 5-8 in his career as a starter, did not throw in practice this week after bruising his throwing shoulder in last week's 51-41 loss to Maryland.
Ehlinger had practiced with the first team all week but wasn't announced as the starter until an hour before the game started.
"It's such a surreal moment for me. Being at the games growing up, literally growing up, maturing in that stadium, literally from when I could walk until now, it was a great feeling," Ehlinger said.
Wide receiver Jerrod Heard served as the backup to Ehlinger against the Spartans. Heard is a former quarterback who started much of the 2015 season when Texas finished 5-7.
Buechele, who threw for 375 yards and two touchdowns last week, set a Texas freshman record last season with nearly 3,000 yards passing and 21 touchdowns.
Coach Tom Herman, who picked up his first win with the Longhorns, wouldn't project his starter for next week.
"Sam played good for that being his first collegiate start. I thought he was very poised in the pocket," Herman said.
This sentiment was echoed by Chris Warren III, who ran for 166 yards and two touchdowns on Saturday.
"I like what I saw from Sam," said Warren. "I'm proud of the kid."
Ehlinger and Buechele were roommates at the team hotel Friday night.
"Shane and I are great friends. He told me just play football, relax and do what you do. His advice really helped," Ehlinger said.
The Associated Press contributed to this report.