The South Korean international was given permission to move to the North-East outfit under former manager Michael Laudrup.
However, following some impressive performances for the Black Cats, Monk is keen to welcome back the midfielder should he remain in charge of the Swans at the end of the season.
"He has put in some great performances up there and he's been scoring goals, which has not been great for us," the South Wales Evening Post quotes Monk as saying.
"In a way it's good to see, but in another way you don't want to see it. I think Ki could potentially a great player for Swansea, but of course we will have to see how he feels at the end of the season."
The 25-year-old, who is said to be wanted by Sunderland on a permanent basis, joined Swansea from Celtic in 2012.