The nineteen year old central defender has recently broken into Chris Coleman's first team and the England Under 21 squad and will go straight into the reckoning for Sunday's match against Celtic.
Kilmarnock have been searching for a central defender since the summer, but short-term solutions have always been hampered by a lack of fitness, so Jefferies will welcome a player who has seen first team action in recent weeks, both in the Premiership and the FA Cup.
He will be particularly relieved to have brought in defensive cover ahead of Sunday's live TV game as David Lilley and Freddy Dindeleux are both doubtful while Gordon Greer remains suspended.