Schalke were victorious in Sunday's Bundesliga Revierderby at the Veltins-Arena, with the hosts winning 2-0.
Belgian striker Michy Batshuayi could be in danger of missing the World Cup, after suffering a reported ligament damage to his ankle during Borussia Dortmund's 2-0 defeat at Schalke on Sunday.
Second-half goals from Evgen Konoplyanka (50) and Naldo (82) handed Schalke their 16 win of the league season which sees them 20 points behind leaders Bayern Munich.
With four games to play, qualification for next season's Champions League is by no means a given, and Reus revealed that only the end goal - qualifying for the competition - and not the means is what counts now.
Coach Domenico Tedesco celebrated with fans in the stands.
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Schalke remain second but are now four points above fourth-placed Dortmund.
Schalke looked more likely to score in the first half, though Dortmund's Marco Reus went closest when he drew a good save from Ralf Faehrmann with a free kick. The defender hammered the ball past Dortmund's ineffective wall and inside the left post again.
Konoplyanka broke the deadlock five minutes after the break after a mistake from Marcel Schmelzer.
The striker has been in fine form for Dortmund since joining on loan from Chelsea in the January transfer window; he failed to break into the side on a regular basis at Stamford Bridge.