His brother, ex-NFL player Chris Gronkowski, says maybe ... 'cause the older Gronk tells TMZ Sports neither he nor the Patriots superstar is sure of Rob's NFL future beyond Feb. 3.

"I couldn't tell you, and I don't even think at this point that he knows what he's going to do after the season."

Gronk's no stranger to retirement rumors ... he's dealt with a TON of serious injuries in his 9-year career -- and even reportedly threatened to walk if he was traded before this season.

But, the dude is only 29 years old ... and is still a beast -- which Chris says people are starting to take note of again now that fantasy football seasons are over.

"If you actually watched him play throughout the year, he was crushing people all year!"

Sooo ... could there be only four more quarters of that in his legendary career???