Watson, who had originally planned to retire after the 2018 season, explained the situation in detail on Facebook.
"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 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."
The Patriots signed Watson to a one-year, $3 million contract on May 9.
"I am excited and thankful to return to New England but very disappointed that I will not be able to play and contribute immediately," the 38-year-old Watson wrote on Facebook.
"This is not how I would want to enter a new locker room and attempt to earn my role on a new team. However, I respect the regulations that have been collectively bargained to promote fairness on the field of play and accept the discipline associated with my infraction."
The Patriots are in transition at tight end after the retirement of Rob Gronkowski, and Watson is a top option to fill his void.
Watson will be heading into his 15th season in the NFL. He had 35 catches for 400 yards and two touchdowns with the New Orleans Saints last season.