The 32-year-old Nelson has been San Francisco's long snapper since 2014. He entered the NFL as an undrafted free agent out of New Mexico State in 2011 and bounced around between several different teams before finally catching on with the Washington Redskins in 2013.
After spending part of one season as Washington's long snapper, Nelson joined the Niners.
Nelson violated the NFL's PED policy as a rookie in 2011, and with regard to his second violation, he denied taking any performance-enhancing drugs knowingly:
"I have been taking the same supplements for my entire time with the 49ers, while undergoing often simultaneous testing by the NFL with no issues. I do not deviate from my nutrition routine, and I would never knowingly take a banned substance.
"I am currently having the supplements tested, and I hope to uncover the cause of the positive test. When the product is identified I fully intend to seek legal action against the manufacturer to protect against this happening to other players in the future."
While Nelson was out last season due to suspension, Colin Holba filled in as San Francisco's snapper, and he is likely in line to do so again in 2019 for six games until Nelson returns.