Reigning Premier League champions Liverpool are in the midst of a surprising slump, given that they bolstered their squad over the close-season to the tune of nearly £450 million worth of new signings.
Arne Slot's team lost 2-1 to arch-rivals Manchester United on Sunday, coming after defeats against Crystal Palace, Galatasaray and Chelsea. Liverpool had not lost four in a row for more than a decade.
Speaking to reporters on Tuesday, Toppmoller said that Sunday's game "could have been 4-2 for Liverpool", adding that the Reds "are not in top form, only in the sense of their results.
"When you watch the games, which of course we did, you see that they seem to lack a bit of a spark at the moment," the 44-year-old said.
"But in every game, they've been the better team and have had more chances at goal.
"We're expecting a world-class team tomorrow," Toppmoller said, adding that the English champions would arrive in Frankfurt highly motivated.
"You've got to be aware of what it does to a team to lose four in a row."
Liverpool's last four-game losing streak came in November 2014 under former manager Brendan Rodgers.
One of the team's issues is the dip in form suffered by star forward Mo Salah.
But Toppmoller said that the Egyptian "was probably among the top three players in the world over the past decade - and still is."
Another Red not delivering his best is Germany's £100 million midfielder Florian Wirtz, who is returning to his home country for the first time since moving to Liverpool from Bayer Leverkusen in the summer.
Toppmoller said that Wirtz's performances were "quite decent, even if he hasn't got the numbers he had in Leverkusen.
"He's at a club where the spotlight is so bright and where people across the world are always watching.
"I'm incredibly convinced of his quality. I hope he doesn't show that tomorrow, that's something we should worry about."