The Scottish champions lost 3-2 in the first leg of their last-16 encounter leaving the Catalans firm favourites to go through to the quarter-finals in the return meeting at the Nou Camp on March 4.

Lionel Messi scored either side of a wonderful Thierry Henry goal to give the visitors the advantage but the narrowness of the scoreline masked the gulf in class between the two sides.

Barca coach Frank Rijkaard was able to take off Deco, Ronaldinho and Henry after the break and bring on Xavi, Samuel Eto'o and Eidur Gudjohnsen.

"They were even better than I had thought they would be," the 24-year-old Australia international said.

"You just had to look at their bench to see what they could bring on.

"It's not every team that can bring on Gudjohnsen, Xavi, and Eto'o when a few of their other stars get tired.

"It was frightening to look at.

"They are exceptional and if we don't beat them then they will go very close in the competition."

McDonald, who has already scored 24 goals in his first season at Celtic Park since arriving from Motherwell last summer, is well aware of the task awaiting the Hoops in the second leg.

He said: "I wouldn't say a miracle (was needed to get through) but it will take a lot of hard work, belief and ability.

"But as the gaffer (Gordon Strachan) said the other day, you wouldn't put your house on it.

"However, we have got to go there with a positive attitude.

"We were disappointed that we didn't come out with a positive result on Wednesday.

"We kept giving the ball away cheaply which made it difficult for us but there was a lot to take from the game.

"It will be difficult going to the Nou Camp but it is exciting.

"I don't think any of us have played there before so it is something to look forward to and it will be a fantastic occasion."

Celtic turn their attention back to the Clydesdale Bank Premier League on Sunday when they take on St Mirren at Love Street.

The champions are four points behind SPL leaders Rangers who should take all three points against bottom side Gretna on the same day.

The former Fir Park player believes the three remaining Old Firm games will decide the destination of the title this season.

He said: "Rangers are four points ahead and probably rightly so.

"We have to go and catch them this year.

"There are plenty of games to go but I think it will go down to the three Old Firm games ahead of us.

"I really believe that both Old Firm teams are not going to give much away until they play each other.

"But we are more than confident that we can do it.

"We have been playing well over the last two or three weeks.

"The new players that have come in have given us a boost and hopefully that continues on Sunday and we can get another good result."