McCown was 26-for-42 with 258 yards and a touchdown and interception. He was also sacked five times by UCLA.
"Frustrating day,'' coach Karl Dorrell said, adding that he did see some encouraging moments out of McCown. "He did some really positive things, and he did some rookie things, too.''
McCown is Colorado's third different starting quarterback, as the team tries to spark an offense that, entering Saturday, had scored only 30 points in three blowout losses and ranked 124th nationally in pass offense. Returning starter Brendon Lewis opened the season as CU's starting quarterback but was replaced by Tennessee transfer J.T. Shrout in Week 2. Shrout started last week against Minnesota but rotated with Lewis, as neither had much success in a 49-7 loss. McCown came in and passed for a team-high 52 yards in the game.
The 6-foot-2, 175-pound McCown was an ESPN three-star recruit out of Rusk, Texas. Josh McCown, who played college football at SMU and Sam Houston State, played 18 NFL season and had 17,731 passing yards and 98 touchdowns.
Saturday's loss capped a difficult week for the Buffaloes and their third-year head coach whose team suffered through a brutal non-conference schedule that led to athletic director Rick George issuing a statement hoping to quell fans' frustration.
"I want our fan base to understand we're putting in a lot of work and effort,'' Dorrell said. "Some of it is just youth. I hate to use that as an excuse, but they have to learn.''
The Associated Press contributed to this report.