Could Super Bowl Sunday be the last time Rob Gronkowski hits the football field??
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.