Shackleton spent last season on loan at Millwall and the deal had an option to buy if the Lions got promoted to the Premier League.
However, the London club finished just outside the Championship playoff spots and the player returned to Leeds with the expectation of leaving the Whites in the summer.
He was one of the players who was expected to move on from the Yorkshire giants in the last transfer window.
It has been claimed that Scottish giants Rangers considered the possibility of getting their hands on Shackleton.
Rangers boss Michael Beale is known for his knowledge of players in the Championship and Shackleton was on his radar.
However, their interest never developed into something concrete and Rangers never made a move for him.
QPR were also interested but Shackleton had already impressed Leeds boss Daniel Farke during pre-season.
He is now an important member of the Leeds squad and he has featured six times for the Whites this season in four different positions.
