Barcelona are firm favourites to beat Stuttgart on Tuesday night, but sporting director Txiki Begiristain believes it will be a tough test.
Speaking to Marca, Begiristain said, "I am conscious of how difficult it will be."
"We will try and play our style. Stuttgart have improved a lot since the draw.
"Against Lyon last season, we played very badly in the first half hour. We must concentrate hard.
"Otherwise, they will cause us problems."
The Blaugrana recently suffered their first league loss of the season, against Atletico Madrid.