Can you imagine an NBA ref chasing down Zion Williamson for an autograph after a game??

That's pretty much what happened after a Champions League match on Tuesday ... when a ref ran after 20-year-old Borussia Dortmund star Erling Haaland -- and got him to sign his yellow card!!

The whole interaction was caught on video after Dortmund's 1-2 loss to Manchester City ... with assistant ref Octavian Sovre tailing the striker as he headed to the locker room.