Benjamin Watson wrote on Facebook on Sunday that he is facing a four-game suspension for a failed test in March of the NFL's policy on performance-enhancing drugs.
The tight end recently signed a contract with the New England Patriots.
He had originally planned to retire following the 2018 season before signing a deal in May.
"After my contract expired last March, I told my doctors I was finished playing, went through a series of medical tests and was prescribed Bio Identical Testosterone Cypionate to assist in healing my body and mind," he wrote on social media.
"On March 29, nine days after I started therapy, I was randomly tested under our substance policies. I complied out of habit, never thinking in the moment I'd want to come back. In late April, some clubs expressed interest in me playing and after much deliberation and prayer, I decided I wanted to return. Considering myself previously retired, I had forgotten all about my test in March until I got a letter on May 3rd saying my results were positive. I was devastated and for obvious reasons did not want to proceed.
"At that point I knew my decision to return to play would include a four-game suspension and I immediately discussed this new development with the clubs."