Maybe ... the 6-time Pro Bowl wide receiver tells TMZ Sports, "We'll see what happens, man ... We'll see what happens."

35-year-old Marshall just had a workout with the Indianapolis Colts a few weeks ago (he said it went well) -- but it doesn't seem he's getting a whole lot of interest from teams beyond that as the season kicks off this week.

So, we had to ask ... is retirement coming???

Marshall wouldn't say ... only telling us he's happy with his roles on "Inside The NFL" and in the fitness industry.

"I'm doing what I love to do right now," Marshall says. "I've been doing that for some time now. And, there's always a lot to work through."

If it is indeed the end of Marshall's career ... it sure was a good one -- the guy played 13 years and logged 12,351 receiving yards, 83 TDs and 970 catches.