The NFL said in a statement that Ninkovich is eligible to return from his suspension, which is without pay, on Monday, Oct. 3.
Ninkovich told ESPN on Friday that he is devastated and shocked, and said he would never knowingly take an illegal substance.
"Few things are more important to me than my name and reputation. This might call that into question for some, which has me heartbroken," Ninkovich said. "I don't want to cut any corners. I want to do things the right way, with high integrity, and that's what I have always wanted to stand for."
"Few things are more important to me than my name and reputation. This might call that into question for some, which has me heartbroken."Rob Ninkovich
Ninkovich, 32, has played in 116 consecutive games (including playoffs) for the Patriots since signing with the team as a free agent on Aug. 2, 2009.
"Any supplement I've ever used was bought at a store. I was unaware something I bought had a substance in it that would give me a positive test because it wasn't listed [as an ingredient]," Ninkovich told ESPN.
"One thing I have learned is that if a supplement is not NSF certified, there are no regulations that ensure that what is on the label is 100 percent accurate. That is a hard lesson for me to learn at this stage in my career, but I take responsibility for it. It's a mistake I made and it hurts that I won't be there for my teammates."
A team captain in 2015, Ninkovich tore his triceps during a training camp practice with the New Orleans Saints on Aug. 9 and was in jeopardy of missing the start of the season because of the injury.
He is valued for his pass-rush ability (47 career sacks, including playoffs) and toughness to set the edge against the run. Coach Bill Belichick has annually lauded Ninkovich's toughness, versatility and intelligence.
The Patriots project to start veteran Chris Long, who signed with the Patriots after spending the first eight years of his career with the Rams, in his place. Jabaal Sheard is the other starter, with Shea McClellin (fifth year) and second-year players Trey Flowers, Geneo Grissom and Rufus Johnson rounding out the depth chart.
"My focus is on getting back to 100 percent and being the best teammate and Patriot I can be upon my return," Ninkovich said.