The Republic of Ireland striker's departure leaves United one man light in the striker department.
Pardew told the Evening Chronicle: "Leon did a great job for me.
"But the thing about Leon is that he needed regular first-team football."
And Pardew insists he could not guarantee the Nottingham-born centre-forward first-team action.
He said: "You can only play a role where you're in and out of the team for so long.
"I felt it was right for him, and everybody concerned, that he moved on."