The Bundesliga side narrowly defeated struggling Braunschweig on Saturday and the club's general manager insists they must up their game in midweek
The Gelsenkirchen outfit narrowly defeated bottom club Eintracht Braunschweig on Saturday, coming from 2-1 down to eventually claim a 3-2 win to move them up to fifth place in the Bundesliga table.
Jens Keller's side currently top Champions League Group E and can take a significant step towards qualifying for the knock-out stages with a win over Chelsea, but Heldt has warned they must improve to stand a chance against the Premier League giants.
He told reporters: "If we want to compete against Chelsea, we definitely need a boost in all individual performances.
"There were hardly any phases we really played football [against Braunschweig].
"Some players really did not convince me. But no names... we'll deal with all that internally."
Schalke will be hoping to welcome back Kevin-Prince Boateng for the meeting with Jose Mourinho's men at the Veltins Arena, with the Ghana international having missed out on Saturday due to a knee injury.