Celtic reached the Champions League round of 16 during the 2012/13 campaign, beating Barcelona in the group stages.
Celtic sent shockwaves across Europe with their Champions League success 9 years ago and Sir Alex Ferguson had a role in the Hoops' success.
Speaking back in 2012, then-manager Neil Lennon said: "Sir Alex gave us two hours of his time and it was a brilliant gesture. It was in the privacy of a room at the training ground, just myself and my backroom staff, some tea and pancakes.
"He was in great form. He has given us little titbits along the way. I'm not saying that has got us over the line but every little bit helps. He was one of the only managers to write me a letter when we won the league in May and I'll be eternally grateful."
Lennon saw his side's Champions League adventure come to an end in the Round of 16 where they were beaten 5-0 on aggregate by Juventus.