Walter Smith revealed losing out on the Scottish Premier League title was even more painful because he believed not so long ago that Rangers would be crowned champions.
The Ibrox club had been odds-on favourites for the championship following a successful run of games which had seen them in the hunt for an historic quadruple just a few weeks ago.
But, just over a week after missing out on the UEFA Cup to Zenit St Petersburg, Rangers were forced to settle for the runners-up spot in the SPL as Celtic completed a hat-trick of titles.
Superior goal difference meant Gordon Strachan's men headed into the final night with the advantage and they produced the goods against Dundee United with a 1-0 win at Tannadice, while Rangers slumped to a 2-0 defeat at Aberdeen.
Asked to sum up his feelings afterwards, Smith said: 'At the present moment, just disappointment.
'I felt at one stage of the season we had a good opportunity to win the league. That was more than I would have expected in the first year but it was there so the disappointment is even greater when it doesn't materialise.
'We are obviously disappointed with the outcome tonight and obviously disappointed that we have lost out on the championship.
'There wasn't much between the two teams in the game and we were disappointed to lose from two set plays - I think it was mental tiredness as much as anything else.'
Goals from Lee Miller and Darren Mackie sealed the win for the Dons but Smith believes a gruelling fixture schedule - this was their 67th game of the season - ultimately proved to be Rangers' downfall even before they made the trip to Pittodrie.
He said: 'I don't think it was a matter of when it started to go wrong - it was just a period of time when we realised what kind of fixture list we were going to have towards the end of the season in the last couple of months.
'That was the realisation after we had had a really good run of games where we went over 20 games without losing. I thought then we could maybe go on and take the championship because of that level of consistency.
'But, once I saw all the fixtures, I knew it would be an uphill battle and that's the way it was for us. That's where we have fallen down as much as anything else.'
However, Smith refused to lay the blame with the Scottish Premier League for their refusal to extend the season further, adding: 'Any other club in our circumstance would have tried to have had the fixtures spread out but when that doesn't happen, you've got to play the games.
'So we have played the games and it's a set of circumstances that don't normally come around and this season it obviously hasn't helped us with the way it has turned out.'
Meanwhile, Aberdeen boss Jimmy Calderwood insists the wrong half of the Old Firm ended up as Scottish champions tonight.
He believes Rangers' crippling schedule, combined with a Zander Diamond goal wrongly chalked off to hand Celtic victory at Parkhead recently, were the key factors in deciding the destination of the trophy.
'Of course I feel for Walter and (assistants) Ally (McCoist) and Kenny (McDowall),' he said.
'Taking nothing away from Gordon's title but I honestly believe that, if Rangers didn't have the programme that they had, they would have won the league.
'But it can be very close. Celtic maybe deserved to beat us that day but, if that goal had stood, I think the league would have been finished that day and the league would have gone somewhere else.'