LOS ANGELES -- Mike Thomas, the Los Angeles Rams' second-year receiver, has been suspended without pay for the first four games of the 2017 season for violating the NFL's policy on performance-enhancing substances, the league said Friday.

Thomas will be eligible to participate in all preseason practices and games, but won't be eligible for the Rams' 53-man roster until Oct. 2, after the team's Week 4 road game against the Dallas Cowboys.

A sixth-round pick out of Southern Miss, Thomas barely played as a rookie and committed two notable mistakes when he received opportunities down the stretch -- fumbling the opening kickoff against the Falcons, and then dropping a potential long touchdown against the Seahawks. But as one of few true deep threats on the roster, Thomas seemed to be carving out a role in head coach Sean McVay's offense and appeared to have a chance to make the team out of training camp.

During the offseason program, Thomas said he has undergone "tremendous growth" from his first to his second year.

"My rookie year, I was a little hesitant," Thomas said then. "Just being at a new level, new environment, new coaches, I wasn't feeling myself as far as playing fast. But now I know what to expect."

Now, though, Thomas will sit out the first one-fourth of the Rams' season.

He is the second Rams player to be suspended for the start of the season. Cornerback Troy Hill was dealt a two-game suspension for violating the league's substance-abuse policy. The NFL says players are handed four-game suspensions on their first "positive test result for stimulant or anabolic agent."

No specifics were provided as to which substance prompted Thomas' failed test.