The Hoops were stunned by Lincoln Red Imps, and Tierney has discussed how the disappointment ranks in comparison with last term's defeat to Glasgow Rangers.
Celtic's Kieran Tierney
Kieran Tierney has suggested that Celtic's humiliating defeat to Lincoln Red Imps in the Champions League ranks right alongside the side's loss to Rangers in last season's Scottish Cup semi-final, The Scottish Sun reports.
The Hoops go into this week's second leg needing to overturn a one-goal deficit after a stunning defeat in Gibraltar. And while many expect Brendan Rodgers' men to comfortably progress to the next round even after the loss, it is unlikely that the result from the first leg will be forgotten any time soon.
Celtic manager Brendan Rodgers
And Tierney has admitted that it will go down alongside their defeat to the Gers last term as the greatest disappointment he has endured since he broke into the team.
"That was the lowest I've felt. Was it worse than losing to Rangers? Neck and neck, I would say," he told The Scottish Sun.
"You can't lose either of those games, you hate to be beaten in either of them. But, in this case, it was who we lost to as well."
Rangers players celebrating winning the penalty shootout against Celtic
Although the defeat to Mark Warburton's men saw them knocked out in the semi-final by their bitter foes, who were in the division below when they met, there can arguably be little question about which of the two losses will be remembered for a much longer time.
While Rangers were in the second tier at the time, they still boasted a squad that had beaten top flight sides fairly comfortably on their way to the last four, and still required a penalty shootout to get through to the final. However, Lincoln Red Imps arguably had no right to win their game.
And no matter what Brendan Rodgers goes on to achieve in his time at Parkhead, he will never be able to shake off the fact that in his first competitive game in charge, they were stunned by a semi-professional outfit in one of the greatest footballing upsets in recent memory.