"I think we need a lift at quarterback. We need a spark," Trestman said, adding that the "weight of the world shouldn't be on the shoulders of Jimmy Clausen."
Trestman said he discussed the situation with general manager Phil Emery, but the call to send Cutler to the bench was "ultimately my decision."
Cutler said he was "shocked" at first at the benching and then "disappointed."
"I didn't see it coming," he said.
Trestman said the benching is just for this week at this point and that Cutler would be active and serve as the backup in Sunday's game against the Detroit Lions.
"We're not playing well enough at the quarterback position on a consistent basis," Trestman said.
Asked if he could play for Trestman next season, Cutler said, "Yes ... but we'd have to work on some things."
Cutler has struggled this season in leading the Bears to a 5-9 record.
In Monday's 31-15 loss to the New Orleans Saints, he threw for two touchdowns but had three interceptions to bring his NFL-leading totals to 18 interceptions and 24 turnovers. He produced a season-low passer rating of 55.8 and generated a total QBR of 6.8, which registered as his second-worst performance of the season in that category (6.0 QBR in Week 10 against the Green Bay Packers).
He also was sacked a season-high seven times against the Saints.
Cutler's contract could be a big reason for the quarterback's benching, according to ESPN NFL Insider Mark Dominik.
The deal calls for Cutler to be paid $15.5 million in 2015. A $10 million guarantee kicks in on March 12, the third day of the 2015 league year, if Cutler is on the Bears' roster. If Cutler is injured at that time, the Bears will owe him $10 million in 2016.
By sitting Cutler the final two games of the season, the Bears guarantee he won't get injured and will have the ability to cut or trade him after the season, basically saving the team $10 million.
Cutler was asked where he would like to play next season.
"I would like to stay here," he said. "Looking forward, this is the place I want to be."