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Oakland Raiders cornerback Nevin Lawson announced Monday he has received a four-game suspension for a failed drug test.

He explained the situation on Twitter Monday, saying he tested positive for Ostarine despite "never knowingly" taking the substance:

Lawson started 45 games over the past three years with the Detroit Lions before signing a one-year, $3.05 million deal with the Raiders in March.

Lawson finished last year with 53 tackles, good for fifth on the Lions, but has struggled to put up numbers in the passing attack. He had only five passes defended last season and doesn't have a single interception in his career.

However, he is expected to play a significant role this year with Oakland as a difference-maker in the slot. His experience was especially valuable in a secondary that is filled with younger talent.

Unfortunately, the 28-year-old will now miss the first four games, which could be a huge blow for a team that finished dead last in the NFL in points allowed last year.

Gareon Conley, 24, suddenly becomes one of the few experienced players on the roster at cornerback with only safety Lamarcus Joyner capable of providing much veteran leadership in the defensive backfield.

Rookie second-round pick Trayvon Mullen will likely see additional snaps while Lawson is unavailable.