Despite being on a much slower pace, he's doing his best to be a team player this season.
Stacy is healthy, but he's dropped to third in a rotation that's been whittled to two the past few weeks.
Stacy's lack of participation was glaring last week at San Francisco. He was the only player the Rams (3-5) didn't use in a 13-10 victory.
''It's just a unique situation, a very strange situation,'' Stacy said. ''At the end of the day it's a business. Some players tend to forget that, but that wasn't the case with me.
''Right now I'm still here. Obviously.''
Coach Jeff Fisher said it was ''one of those things that happened.''
Moving forward, he's making no promises and added it was ''probably unlikely'' Stacy could find an outlet on special teams.
''I can certainly appreciate how Zac feels after last week,'' Fisher said. ''Zac is again, very unselfish and just waiting his turn, and he's an outstanding back as well.''
Stacy was a standout last year as a fifth-round pick off a record-setting career at Vanderbilt, maximizing his chance after getting his first start in Week 5. He got banged around so much on game days the Rams typically held him out of practice until later in the week so he'd be full go when it counted.
Nowadays, there's a lot of downtime for a player who was a workhorse as a rookie, getting 26 or more carries in five games.
''First string or fifth string, you've really got to be professional about yourself,'' Stacy said. ''My whole mentality is remaining prepared as much as I can.''
Mason was a third-round pick after being a Heisman Trophy finalist for national runner-up Auburn, where he was SEC offensive player of the year.
Mason was held back early after some struggles on pass protection in the preseason and Cunningham still often replaces him in third-down situations, but he gets most of the handoffs.
Stacy leads the team with 257 yards to go with a 3.9-yard average. But in the past four games, he had only 34 yards on 13 carries. Mason has 222 yards with a 4.5-yard average, and had 19 carries for 65 yards against the 49ers.
Cunningham, undrafted and in his second season with St. Louis, has been a multifaceted option. He had a 75-yard kickoff return in Week 7 against the Seahawks, is third on the team with 20 receptions to go with an 8.4-yard average and third in rushing with 173 yards with a 3.9-yard average and two TDs.
Quarterback Austin Davis said he doesn't mind who is in the backfield.
''No, we work all those guys,'' Davis said. ''We work on consistency. They're all in the same places, for the most part. It's not really an issue at all.''
A day after the victory, Fisher was unhappy about Mason getting tackled by the 49ers' Chris Borland in the fourth quarter and not getting whistled for a personal foul.
Mason avoided injury and has been a full participant in practice.
Whenever they need him, Stacy's ready.
''One thing about it, I've got a good grasp of the offense, so in the back of my mind it's still tuned in,'' Stacy said. ''Whenever I'm in the game, I'm able to process things a whole lot better - even without the reps.''
AP NFL website: www.pro32.ap.org and www.twitter.com/AP-NFL