Gordon Strachan admitted Oscar Cardozo's late winner was hard to take after his side's gutsy Champions League display in the Stadium of Light.
The Paraguayan international striker twice hit the woodwork and was denied by a brilliant save from Celtic keeper Artur Boruc before netting the 86th minute goal which secured his side their first points in Group D.
Under the cosh
Celtic had more than matched the Portuguese giants in the first-half but found themselves under the cosh in a one-side second-half before succumbing late on.
The result now leaves Celtic with it all to do if they are to secure qualification to the knockout stages this season.
Strachan told skysports: "You have to handle these blows. It was terrific heart that the players showed today.
"In the Champions League that helps and quality passing helps you as well, we chased the ball a lot and we had tired minds at the end."
The result extended Celtic's miserable away form in the group stages to 14 games without a win but midway through the second-half the Celtic boss felt that run was coming to and end.
"We had a couple of chances ourselves and on a lucky night I thought we might
get the result," he added.
"When the guy (Cardozo) had the turn and hit the post I thought it was going to be our night but it just shows you it can be a very cruel game and you have to deal with that."