He follows teenage centre-back and England Under-21 international Cresswell to The Den where he will work under Gary Rowett.
South London Press report Shackleton is due for a medical with his loan move set to be all tied up this week.
Unlike Cresswell’s straight loan deal, Shackleton’s switch could be made permanent next summer in a similar situation to Tyler Roberts who joined fellow London side Queens Park Rangers on loan earlier this window.
Shackleton’s Leeds deal expires in 2024 and has racked up 78 appearances across all competitions for his boyhood club since joining United at the age of seven.
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South London Press report Shackleton is due for a medical with his loan move set to be all tied up this week.