The Northern Irishman's appearance in Saturday's 4-1 victory at Scunthorpe United was his seventh start in the Championship for Leeds.
And Grayson is confident that there is more to come from the 29-year-old if he plays on a regular basis.
"We were keen for George to stay with us for a longer period and I'd like to thank Steve Bruce at Sunderland for agreeing to that," Grayson told the club's official website. "I do feel there is a lot to come from George and the more games he plays the more people will see."