FRISCO, Texas -- Dallas Cowboys defensive end David Irving is facing a four-game suspension for violating the NFL's policy against performance-enhancing drugs, a source has confirmed.

An appeal on Irving's behalf was filed last week. A source said the issue stems from an over-the-counter substance Irving used in an attempt to endorse the product.

Irving started two of 16 games last season, but he was the Cowboys' most productive defensive lineman down the stretch. He finished the season with four sacks, five tackles for loss and a team-high 26 quarterback pressures. He also had five pass deflections and forced four fumbles, including three against the Green Bay Packers that helped him earn NFC Defensive Player of the Week honors.

In the final four games, Irving had six of his 11 tackles, according to the coaches' breakdown, with three sacks, 10 quarterback pressures, two pass deflections, a tackle for loss and a forced fumble.

He signed his exclusive free-agent tender earlier in the offseason and is set to become a restricted free agent after the 2017 season.

The Cowboys hope Irving can continue to develop and show consistency as he enters his second full season with the team. He joined the Cowboys early in the 2015 season off the Kansas City Chiefs' practice squad.

The pending suspension was first reported by 105.3 The Fan in Dallas.

If the suspension is upheld, he would be the Cowboys' second defensive lineman penalized by the league this year. Randy Gregory will miss the entire regular season for multiple violations of the NFL's substance abuse policy.