Kansas City Chiefs offensive tackle Donald Stephenson was suspended by the NFL on Friday for the first four games of the season. He was suspended for violating the NFL’s performance-enhancing substance policy.

The substance that Stephenson tested positive for was not disclosed. The Chiefs issued a statement saying that they have been informed of the positive test, and that they are disappointed in losing Stephenson and have no further comment. Stephenson said that he took medication that requires a therapeutic-use exemption and failed to investigate it properly, according to the Associated Press.

''The rules are strict, and without a TUE, the consequence is a four-game suspension,'' Stephenson said in a statement. ''It is not a mistake I will ever make again. I will train extremely hard during my time away from the team and I will be ready to contribute to my team's effort to win a Super Bowl as soon as I return.''

Stephenson was a fill-in starter last year. He was expected to be playing right tackle this year after Eric Fisher moved back to playing left tackle. The Chiefs drafted Stephenson in 2012. He has played in all 32 regular season games and started 14 of those games since he was drafted, according to CBS Sports. He started seven games last year and in 2012.

Stephenson is the second Chiefs player to be suspended for the start of the season. Wide receiver Dwayne Brown will miss the season opener after being suspended for one game for violating the NFL’s substance-abuse policy.

Despite the suspension Stephenson is still eligible to participate in preseason practices and games. He will be eligible to return to the active roster on September 30. Ricky Henry and Jeff Linkenbach will compete to replace Stephenson in the first four games of the season.