Schalke host Mainz in one of eight DFB-Pokal third-round ties in midweek
Schalke host Mainz in the DFB-Pokal third round on Tuesday, Bayern Munich host Augsburg and there are six other ties.
Last Updated: 17/12/12 4:39pm
Jens Keller, who has stepped up from coaching the under-17s to replace the sacked Huub Stevens, will take charge of the Royal Blues for the first time with a place in the cup quarter-finals at stake.
Schalke have lost four and drawn two of their last six Bundesliga games, with the latest setback, a 3-1 home loss to Freiburg, proving costly for Stevens.
But Heldt has told the players they also have to shoulder some responsibility for the dismal run and now must raise their game.
He told the club website: "Before Christmas we have another tremendously important game against Mainz in the DFB-Pokal.
"The onus is on the players now. I expect everyone to conduct themselves in a professional manner."
Keller has been handed the reins until at least the end of the season, but on Monday Mainz coach Thomas Tuchel was forced to deny speculation linking him with the Schalke job.
Bayern Munich finish what has been a very impressive first half of the season at home to Bavarian rivals Augsburg on Tuesday.
The runaway league leaders are 33 points better off than their opponents in the Bundesliga, but captain Philipp Lahm warned they cannot afford to take their final game before the winter break easy.
"We'll have to dig deep and give our all one last time," he said.
Midfielder Javi Martinez should be fit to play after coming off in Friday's draw with Borussia Monchengladbach with bruising around the eyeball.
Holders Borussia Dortmund will aim to complete 2012 without DFB-Pokal defeat when they hosts Hannover on Wednesday.
"It's one we really want to win," chief executive Hans-Joachim Watzke said. "We know how good it feels to get to Berlin and we are looking to cap off the year with a win over Hannover."
Bayer Leverkusen, who needed extra-time to see off Armenia Bielefeld in the last round, travel to Wolfsburg on Wednesday, while Stuttgart hosts second division Cologne.
Elsewhere on Tuesday, Freiburg travel to third division Karlsruhe and Fortuna Dusseldorf are away at another third-tier side, Kickers Offenbach.
Wednesday's final tie is an all second-division affair between Bochum and 1860 Munich.