The Blues are going for the Dane after Liam Delap suffered a serious hamstring injury in victory over Fulham on Saturday.
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The Sun says Chelsea have been defied by Nicolas Jackson who was ordered back from Germany as he prepared to undergo a Bayern Munich medical.
Jackson remains in Munich this morning and Chelsea are now willing to relent - if Bayern are ready to sign him permanently immediately.
Should that happen, Chelsea will move for Harder as a direct replacement.