Knight had injured her lower back during the opening overs of the first ODI of the series in Coolidge, where Australia thrashed West Indies by 178 runs. Knight did not bat in the West Indies chase as a result, and did not feature in the second game. After "examining the scans" done on her, the CWI medical panel ruled Knight of the third ODI.
Cooper joined the team in Antigua on Monday in preparation for the final game even as West Indies trail the series 0-2.
Cooper last played an ODI in June in England but her single-digit scores in the series led to her omission from the original ODI squad against Australia. In six international innings this year - two ODIs and four T20Is - she has scored only 57 runs so far with a high score of 20.
West Indies are also without their regular vice-captain Hayley Matthews, who had been withdrawn from the series just hours before the start of the opening ODI, due to disciplinary issues. The exact nature of her breach of the code of conduct is not known yet.