Gordon, who was reinstated by the league in 2017, announced Monday morning on Twitter that he will not report for camp Wednesday when the rest of his teammates show up.
While he didn't go into particulars, Gordon stated that current treatment, which will keep him away from the practice field, surrounding his well-being has been coordinated with the NFL, NFLPA and the Browns.
"To my Cleveland Browns and NFL family, I am reaching out to you all personally and letting you know that I am not only doing great physically but mentally as well," Gordon tweeted. "You will notice that I will not be in Cleveland for the start of training camp. Rest assured this too is a part of my overall health and treatment plan.
"I appreciate the awesome support I have received from teammates, friends, fans, and the Browns' organization. Just like you, I am excited to start the season and I have every intention of being ready and available to join my teammates soon to help bring winning football to our fans. With the help of the NFL, NFLPA, and the Browns' organization, I have been able to utilize the resources available to me that will ensure my well-being on and off the field. By continuing to follow the plan set up by our medical director and his team and taking this time before this season starts, we believe it will help me maintain the progress I've made for not only today but for many years to come. Thank you all for your patience, love, and support! Go Browns!"
"We will continue to support Josh as he receives the care needed to maintain his progress," Dorsey said, via the Browns' official website. "We are going to respect his privacy while he is away from the team. Josh will be placed on the non-football illness reserve list until he is ready to return."
NFL spokesman Brian McCarthy told NFL Network's Tom Pelissero there is no timetable on Gordon's status.
Gordon entered the league in 2012 as a supplemental draft pick with the Browns, but his career has been mired with multiple suspensions for violations of the league's substance-abuse policy.
He did not play in 2015 and 2016 before returning in November 2017 on a conditional reinstatement. In 40 career games, Gordon has 179 catches for 3,089 yards and 15 touchdowns.
"I was surprised," Thomas said. "I knew that Josh has been on the right track. He's been doing all the right things this offseason. But as anybody who's battled addiction knows, there are times when you need a little reminder. You need to ... get back on the right track. It's not the greatest of timing right now for the Browns, but hopefully he can get his house in order by the time the middle of training camp comes around, and he'll be back out there. And I think the Browns are doing the right thing. They're being patient, they're being understanding and they're trying to help a young man that wants to get the right help and that has amazing NFL talent."