Tillman has not played since tearing his right triceps in the Bears' 21-19 loss to the Detroit Lions on Nov. 10.
He was then placed on injured reserve with the designation to return list, meaning he could not practice for six weeks and could not return game action in eight weeks.
The Bears are 8-6 and if the season ended today, they would be the fourth seed in the NFC.
"Tillman will be unavailable for the rest of the year," Trestman said. "He's not going to be available to practice for the rest of the regular season because he's not far enough in his recovery to do that."
Tillman, 32, will be a free agent after the season. He made just over $8 million in 2013. Zack Bowman, who had two interceptions Sunday's 38-31 win over the Cleveland Browns, has filled in at cornerback the past five weeks in Tillman's absence.