Just days before the first-leg, Robertson received the terrible news that his brother and sister-in-law had died in a road accident in Glasgow. A car had crossed the central reservation and crashed into them.
Hughie had been driving a car that John had recently given him. It was the first car he had owned.
Robertson recalls that Clough could not have been kinder and let him decide whether he wanted to play against Cologne. He said that was despite knowing that reaching the European Cup Final was so important to Clough (who had been cheated out of achieving that at Derby).
"He told me to take as long as I wanted with my family and whatever I decided to do he would respect the decision," said Robertson.
Clough told the Daily Mail at the time: "I gave John permission to go home as soon as he heard about the car crash. I told him he could stay in Scotland or come back and play.
"The decision was left entirely to John, there was nothing else I could do in such tragic circumstances."
After speaking to his family, Robertson decided to play against Cologne, as they all felt that is what Hughie would have wanted. Robbo would fly back the next day to attend the joint funeral.
Clough said Robertson's inclusion was an important bonus. When Cologne had previously watched Forest in two matches, Clough said his side was so bad that Robertson had barely had a kick of the ball.
Robbo would be a key addition - "they don't really know how dangerous he can be."
Clough added: "Cologne are typically German - technically good, disciplined, methodical and efficient. We need to produce something a little unorthodox to throw them. Short of playing Peter Shilton at centre-forward, John Robertson is our best means of the surprise element."
When Robertson scored in the first-leg, he remembered his brother, looked up to the sky and said to himself, 'That's for you, my boy.'
'Supertramp' by John Robertson (with John Lawson) is published by Mainstream.