Stefanski said Phillips, a former starter who finished last season on injured reserve because of a pectoral injury, tore the muscle in the third quarter of the Browns' 17-15 loss to the Washington Commanders.
Stefanski said the latest injury is on the opposite side from last season, and he will require surgery..
"It's the worst part of this game, seeing guys get injured and especially you see how hard they work," Stefanski said. "So it hurts. I hurt for Jacob, and this is why we're teammates and why we support each other."
The Associated Press contributed to this report.