Need is related to Necessary. Necessary is the opposite of Impossible. Impossible is related to Difficult. Difficult is similar to Serious. 
Slight is the opposite of Value. Value is related to Profit. 
Need <--RelatedTo--> Necessary <--Antonym--> Impossible <--RelatedTo--> Difficult <--SimilarTo--> Serious 
Slight <--Antonym--> Value <--RelatedTo--> Profit 