Dortmund were asked by UEFA to play the match on Wednesday night after the fixture at Signal Iduna Park was initially postponed when explosions hit their bus as it left the hotel, with defender Marc Bartra being taken to hospital.
Coach Thomas Tuchel did not want to play the match but Dortmund took the field and were 2-0 down at half-time with Monaco also missing a penalty. But goals from Ousmane Dembele and Shinji Kagawa gave the German side some hope going into next week's second leg.
Speaking to Norwegian television, Sahin explained just how hard it was for the Dortmund players.
"It's hard to talk about it," said Sahin. "It's hard to find the right words.
"I don't know if the people can understand this but, until I was on the pitch in the second half, I didn't think about football."
Read the full article on eurosport.co.uk: 'I will never forget the faces' - haunted Sahin gives emotional interview