It came from the set phrase"人为刀俎,我为鱼肉"
人 other people
为 is
刀 knife or blade
俎 a chopping block
我为鱼肉 鱼means fish 肉 means meat
The whole sentence means “to be fish or meat on somebody's chopping block or under their blade”
英超已沦为欧冠的鱼肉 means " 欧冠 can beat 英超 very easily".