Tampa Bay running back Doug Martin was suspended four games for violating the NFL's policy on performance enhancing substances and said Wednesday he's entering a treatment facility rather than appeal the ban.

The two-time Pro Bowl selection, slowed by a hamstring injury this season after rushing for 1,402 yards in 2015, will miss Sunday's regular-season finale against Carolina.

He also sat out last week's 31-24 loss at New Orleans after being declared inactive against the Saints, though that game won't count against the penalty.

Martin said in a statement that he learned of the suspension last week and that his "initial instinct" was to "appeal the suspension and finish the season with my teammates."

The fifth-year pro said he decided to enter a treatment facility "and receive the help I truly need" after discussing the matter with people close to him, including Bucs coach Dirk Koetter.

Martin averaged just 2.9 yards per carry while rushing for 421 yards on 144 attempts this season.

Ravens' Smith may retire

Steve Smith sounds like a man who's ready to close the door on an NFL career that has spanned 16 seasons and taken him well beyond his 37th birthday.

"Whether you want to or not, you've got to move on," Smith said Wednesday. "It's part of life. The world still turns."

The five-time Pro Bowl receiver will play in his 230th NFL game Sunday when the Baltimore Ravens finish the season against the Cincinnati Bengals.

Standing in front of the media wearing a Darth Vader costume, Smith said he's "about 89 percent sure" it will mark the end of his stellar career.

He used that number because he wears No. 89.

Cassel willing to run

Veteran quarterback Matt Cassel has been lobbying coaches to keep the Tennessee offense the same as it has been with Marcus Mariota behind center.

Yes, even down to letting the quarterback run with the ball.

"I've been learning about the zone read all year," the former Viking said. "I've been watching a lot of it. Now I don't know how many times I'd actually pull it in, but at the same time, I've been lobbying for it for sure."

With Mariota breaking his right fibula in last week's 38-17 loss to Jacksonville, that means Cassel will make his first start of his 12th NFL season in the Titans' season finale on Sunday against the Houston Texans. Running isn't something Cassel has done much of in recent seasons, but he has 264 rushes for 1,010 yards with five touchdowns in his career.

Cassel said he's all in, even if coaches put in the triple option. Then he turned serious.

"I wouldn't anticipate much change in the offense," Cassel said. "We've had a lot of success this year doing what we do, and obviously there'll probably be some elements of our game plan that change just based on my skill set based on what Marcus has to offer."

Etc.

•Receiver A.J. Green (hamstring) won't play in the Bengals' final game Sunday against the Ravens, leaving him 36 yards shy of joining Randy Moss as the only NFL players to top 1,000 yards receiving in each of their first six seasons.

•Panthers' three-time All-Pro middle linebacker Luke Kuechly won't play Sunday (concussion).

•The Jets placed running back Khiry Robinson on injured reserve after he broke his right leg for the third time in 13 months.

•Browns starting quarterback Robert Griffin III remains in concussion protocol and could miss Sunday's season finale in Pittsburgh.

EJ Manuel will start Sunday at quarterback for the Bills, replacing Tyrod Taylor.

•The Hall of Fame game will be played on a Thursday night next August, two days before the induction ceremonies. Previously, the game was played on a Sunday night.