"That's a tough break for us," Oakland coach Jack Del Rio said. "It's really unfortunate. He was having a great camp for us."
Watson vowed in a tweet to return at full strength.
Ruptured Achilles' tendon... I Will be back stronger than ever that's a promise. Main thing is supporting the team right now. #RaiderNation
— Menelik Watson (@MenelikWatson) August 31, 2015
Watson, a native of Manchester, England, was a second-round pick of the Raiders in 2013 (42nd overall) after playing only two years of organized football -- one at Florida State and one in junior college.
He has played 17 games in his two NFL seasons, starting 12.
Information from The Associated Press was used in this report.