Age Hareide says his Helsingborgs side were "better than Celtic at everything" in the first leg and can beat the Scottish champions in Glasgow.
Celtic left Sweden with a 2-0 lead going into the return game of Champions League play-off tie.
"Celtic are favourites of course, but football can be strange on some occasions," said coach Hareide.
"Bigger things have happened than Helsingborg beating Celtic and we have to believe in it."
After Kris Commons volleyed Celtic into a two-minute lead in the Olympia Stadium, goalkeeper Fraser Forster pulled off a string of saves to deny the home side until Georgios Samaras gave the visitors the impetus with a second after 75 minutes.
"To be honest, I thought we had a fair chance to beat Celtic before we played them in Helsingborg," said Hareide.
"I was disappointed that we gave them the chances to score goals and we didn't score ourselves.
"But I seen all the stats in that game and we were better than Celtic at everything, in possession, we ran more than the Celtic players, and we actually played really well apart from the beginning and the end of the game.
"But the vital thing in football is to score goals. You can't take that away from them, they did that.
"They got the goals and we have to chase them - and we will chase them."
Hareide stressed the importance of belief to his side's chances of pulling off what would be a remarkable feat and what he admits would be the best result of his managerial career.
"There is always a possibility and it is always about believing," he said.
"If we were outplayed, or the result showed everything, then we would have probably come across just to play the game, but I thought we were really unlucky.
"I know that Celtic looked sloppy against Ross County and Inverness.
"They have some good players, of course, but if you look at the players we have, we can come at them again.
"I spoke to the players and told them we don't have to think about two or three goals but about one goal because then Celtic have the pressure of the crowd."