'It was heartbreaking': 25-year-old admits he was so upset leaving his old club to join Arsenal
The deal only went through on the deadline day of that summer transfer window, but Tierney has said he knew the move was in the offing.
In an interview with BT Sport, Tierney said that the move made sense for all parties, given the fee involved and the fact he was joining another huge club.
But the left-back also said the move broke his heart at the time, as he departed the club he joined at the age of seven and came through the ranks.
Tierney discusses joining Arsenal from Celtic
He said: “It’s so hard. There wasn’t one moment where I’m thinking ‘right I want to leave Celtic’.
“When I was going through all the tough injuries, I think at Aberdeen away when we won the league, I think I could run at 80 per cent speed.
“If you watch that game back, if you look at me, you think I’m miles off it. Can’t run, can’t do anything, can’t turn. I sat down, thinking I need to get off here but the fans are cheering me. I was right next to them, I was thinking I’ll try again and then five minutes later, I’m back down.”
“It was quite emotional inside and I knew my season was over. I’d heard the rumours and you know when a club is going to sell you, you get a feeling inside you. At the time it made sense for everybody.
“They’d brought someone through the youth academy that they’re proud of and he’s going for a record fee. I’m someone who loves the club, gave everything, my 100 per cent from the day I joined at seven to the day I left.
“I was going on to play for a huge club who believed in me and put a lot of faith in me and backed that up. It was something which worked for everybody but it was the hardest decision of my life.
“I was so excited to go to Arsenal but it was heartbreaking.”
Tierney has had his injury problems and this year, he has had competition from Oleksandr Zinchenko, who plays the role of left-back in a different way.
Mikel Arteta seems to want his full-backs coming inside at times, and that is more natural for Zinchenko than it is for Tierney.
That is a challenge for the Scot, but he is getting to grips with it and the most important thing for him is that he stays fit and on form for as long as possible this season.