Washington waived Thomas on Saturday, and Shanahan contended that the receiver's professionalism was lacking.
"Everybody's got a different mindset on what work means, what dedication means, what commitment means," the coach said Monday. "All I told Devin was if you want to be the best player you want to be, your commitment is going to have to be stronger."
It marks the second consecutive week Carolina has claimed a receiver off waivers. It picked up David Clowney last week from the New York Jets.
The Indianapolis Colts also placed a waiver claim on Thomas, a league source told NFL Network insider Jason La Canfora.
The Associated Press contributed to this report.