Schalke, who return to the Champions League on Tuesday after a four-year hiatus, have been accused of "selling their future" for allowing three rising stars to leave.
The Royal Blues, who finished second in the German league last season, host Porto in Gelsenkirchen in their Group D opener, but are struggling after losing their first three Bundesliga games.
Despite finishing second to Bayern Munich last season, Schalke are already struggling this term leaving head coach Domenico Tedesco under pressure after 15 months in charge.
Ex-Germany midfielder Steffen Freund has criticised Tedesco's boss, Schalke's sports director Christian Heidel, after Leon Goretzka joined Bayern and Max Meyer, both 23, left for Crystal Palace on free transfers last season.
On top of that, centre-back Thilo Kehrer, 21, was sold to Paris Saint-Germain last month for a reported fee of 37 million euros ($42.3 million).
"The departures play a big role. Perhaps one notices now that Max Meyer, Leon Goretzka and Thilo Kehrer were the future of Schalke and they sold it," said Freund, a Euro '96 winner with Germany, told broadcaster RTL.
Schalke are second from bottom in the German league table after losing to mid-table sides Wolfsburg, Hertha Berlin and Borussia Moenchengladbach, where they were beaten 2-1 on Saturday.
They reached the Champions League's semi-finals in 2011, bowing out after defeat to Manchester United, but will be lucky to get out of Group D this season, which includes Lokomotiv Moscow and Galatasaray.
Tedesco says the Champions League offers a fresh start as Schalke make their return since losing to Real Madrid in the last 16 of 2014/15.
"Obviously recent results are playing on our minds, but the Champions League is a fresh start for us and we have earned the right to play these games," said Tedesco.
However, Freund, who retired in 2004 after spells with Tottenham Hotspur and Borussia Dortmund, says the problems are in both attack and defence.
"Schalke have problems scoring goals, they need twenty goal shots to score. You have to compare it with last season - because then they only needed eight shots," said Freund, who spent two seasons at Schalke in the early 90s.
"They also concede two goals per game.
"Last season they still had the second best defence behind Bayern.
"What is quite amazing is that they also let in goals from standard set piece plays."
© 2018 AFP