Thornton (undisclosed) exited practice early Thursday and didn't return, Andrew Callahan of the Boston Herald reports.
Thornton left after catching a deep ball from Mac Jones. The 50th overall pick in last year's NFL Draft, Thornton has had a quiet summer after generating some spring buzz in pad-less practices. Sixth-round rookie Demario Douglas may be passing Thornton on the depth chart behind veterans DeVante Parker and JuJu Smith-Schuster.