The Catalan giants have won all of their European games so far this season but are struggling in La Liga and lost 1-0 to relegation threatened Rayo Vallecano on Saturday.
"I'm confident we can do well this season," said de Boer after the defeat.
"We have a lot of quality and a lot of experience in this team. And I still think we can win the title."
"It was a strange goal," said under pressure van Gaal, "I think Rayo were a little lucky.
"But if we can win against Newcastle United on Tuesday we are almost through to the quarter-finals of the Champions League. If that happens this result will soon be forgotten.
"It is now up to the training staff and the players to raise themselves for this game."
Rumours in the Spanish press suggest that defeat by Newcastle on Tuesday would lead to the unpopular van Gaal being sacked.