Del Rio met with Gabbert and McCown on Wednesday morning, then told other players about the switch at a team meeting. Gabbert will start Sunday against the Carolina Panthers.
The transition was inevitable since the Jaguars selected Gabbert with the 10th overall pick in April's draft. Del Rio had hoped to take it slow with the former Missouri standout, even planning to give him a year to watch and learn behind David Garrard. But Garrard had his worst preseason as a starter and was outplayed by McCown, a career backup.
Del Rio named McCown the starter five days before the season opener. But his ninth career start was a debacle. He was picked off four times, and could have thrown a couple more, and was sacked for a safety -- in just three quarters.
Gabbert replaced McCown to start the fourth, and completed 5 of 6 passes for 52 yards in relief.
But Gabbert struggled at times in the preseason, essentially confirming to Del Rio that he needed more time to learn the offense and read NFL defenses.
The Associated Press contributed to this report.