Signing Harder and selling Jackson is the preferred option of three potential scenarios, with manager Enzo Maresca worried Delap will be out for “six to eight weeks”.
Any deal is contingent on both agreeing a fee and convincing the 20-year-old one-cap Denmark international to move to Chelsea to compete with two other strikers.
Alternatively, Marc Guiu could be recalled from his loan spell at Sunderland, but that would require the clubs to reach an agreement.
There is also the possibility that Jackson is instructed to return to London by the Blues.
The transfer window closes at 19:00 BST on Monday, with Brighton attacker Facundo Buonanotte also expected to join on a season-long loan.
Before the injury to Delap, Chelsea had also made an offer to Barcelona forward Fermin Lopez on Saturday.
They also have a key priority to offload their final few ‘bomb squad’ members in Raheem Sterling, Ben Chilwell, Axel Disasi and David Datro Fofana.
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