Watson, who took the banned substance during his brief NFL retirement this offseason, explained the context behind his use.
"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.
"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 that 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 that my decision to return to play would include a four game suspension and I immediately discussed this new development with the clubs.
"Ultimately I decided to pursue another year and on May 9 the Patriots offered me a contract in spite of these circumstances."
Watson, 38, caught 35 passes for 400 yards and two touchdowns for the New Orleans Saints last season. He is entering his 16th NFL campaign.
He's expected to be the Patriots' No. 1 tight end after Rob Gronkowski retired after nine years.
Watson will miss home games versus the Pittsburgh Steelers and New York Jets and road contests against the Miami Dolphins and Buffalo Bills. The Pats figure to be favored in all four games, but losing Watson is still a blow for a team that lost pass-catching talent in Gronkowski and wideouts Chris Hogan, Josh Gordon and Cordarrelle Patterson.
The Pats could then turn to tight end Austin Seferian-Jenkins, who had 11 receptions for 90 yards in five games during an injury-shortened season for the Jacksonville Jaguars. The 26-year-old caught 50 passes for 357 yards and three touchdowns for the New York Jets the year before.
The good news is that Watson will be available for a brutal second half of the season that includes road games against the Super Bowl LII champion Philadelphia Eagles and 11-5 Houston Texans and a 2018 AFC Championship rematch versus the Kansas City Chiefs. An early rest could even behoove the veteran Watson, who will have less tread on his tires as he enters the final stretch of the year after sitting the first four games.
When he does return, he can tap into the chemistry exhibited between him and Tom Brady when Watson was first on New England from 2004 to 2009. Of note, he caught 49 passes for 643 yards and three touchdowns in 2006 and notched a 73.5 percent catch rate in 2007.
Watson can make his debut on Sunday, Oct. 6, at the Washington Redskins in Week 5.