The NFL suspended Cincinnati Bengals reserve linebacker Darryl Blackstock for the next four games on Tuesday because he used a performance-enhancing substance.

The Bengals signed Blackstock as a free agent from Arizona in the offseason. He has played in all five games, including one start, and has four tackles. The fourth-year veteran said in a statement that one of his nutritional supplements contained an ingredient banned by the league.

‘‘I’ve apologized to my coaches and teammates for an unfortunate mistake,’’ Blackstock said. ‘‘I did not intentionally violate the policy, but I know it’s my responsibility to use only approved nutritional supplements, and I didn’t realize I was taking an over-the-counter product that included a banned substance.’’

The Bengals also re-signed running back Kenny Watson, who was released on Saturday when they needed to open a roster spot for receiver Chris Henry. The troubled receiver returned from a four-game suspension for misconduct and played in a 31-22 loss at Dallas, where he didn’t have a pass thrown his way.