Hahnemann watched from the bench on Saturday as Green let a tame shot from Clint Dempsey slip through his hands and said he felt a rush of emotions.
"At first it was absolute elation, Clint came over to the bench to celebrate and then I sat back down and thought I wish that had gone in the top corner.
"I know Robert Green, he is a great guy and a great keeper and I have played against him for a number of years now and you know what's going to happen in England," he added.
"But Robert is a strong character and he will be fine," added Hahnemann, who plays in England's Premier League with Wolverhampton Wanderers.
Green's mistake was one of several keeping errors in the tournament so far and Hahnemann said keepers never want to see their counterparts make bad errors.
"You never wish anyone luck on the other team except I always wish the other keeper good luck," he said.
"This is why I have a short term memory, so I've never made any mistakes - that's how I deal with stuff. It means I don't remember the good stuff either though," he joked.
"You need that short-term memory to forget things. If you don't have that strong personality you can't go on. For him (Green) to make that save from Jozy Altidore in the second half it shows what he's got," he said.
