Barca warmed up for the game with a thumping win over Racing Santander.
But Messi warned: "At this point nothing is easy, so we have work more than ever to get a good result.
"I scored against Stuttgart two years ago and I believe my game hasn't changed since then. I will continue, as always, to score and help my team win."
Messi also admitted their last two performances against Atletico Madrid and Racing were up to their usual standard.
"Maybe we have not played like we've done, but at this stage usually you have to win either way," he added.