The attacking midfielder, a 42.5 million summer signing, has been in the form of his young life.
The 21-year-old has netted 20 Premier League goals this season to move level with Erling Haaland.
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Per The Sun, his 80,000-a-week, seven-year deal does not include any big goal bonuses.
Palmer will be regretting that decision, but may have a chance to get such a clause in his next deal.
Chelsea are delighted at the way Palmer has grown since arriving in the summer from Manchester City.