The Ligue 1 champions were swept aside in their own stadium last week by a dominant Barca led by a lethal Luis Suarez who grabbed a brace in the 3-1 defeat.
They have Zlatan Ibrahimovic and Marco Verratti back from suspension for the second leg but Thiago Silva joins Thiago Motta among the injured and David Luiz is also a fitness doubt.
"We have to think that we have to score and I am sure that we will but we have only a small chance to go through," Blanc told a news conference in Barcelona on Monday.
"We cannot afford to concede goals and this here is practically impossible."
"All the teams that play here in Europe concede at least one. They (Barca) have attacking players with incredible talent," Blanc added.
He said that whatever fate awaits his team he will not make excuses about players missing.
"Those who are not here were not fit for the game and I wish I could have had them but it is not possible and it is no point feeling sorry about it," said Blanc.
"We have to show that we want to win, show that we are capable of doing it and give a better account of ourselves than in the first match."
Ibrahimovic, currently serving a ban domestically for abusive comments to a referee, returns to the club where he played during the 2009-10 season and coach Blanc hopes it will give him a lift.
"Ibra likes these these kinds of games when there is nothing to lose," he said. "He will help us and I am looking forward to seeing him play.
"He is very important for us and is training hard each day. He is in good form."
(Editing by Martyn Herman)
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