The 20-year-old, who scored the winner for Sunderland in their memorable 1-0 win over Manchester City in the English Premier League on Januray 1, has only made one start for the club since joining from Chunnam Dragons in his homeland in June.

Ji has been a regular off the bench for the Black Cats, and O'Neill told The Sunderland Echo he had rebuffed offers from clubs keen to take the forward on loan as he wanted to keep the 'talent' at the Stadium of Light.

"We've had a few phone calls from clubs abroad that would like to take Ji on loan, but we've knocked them back," O'Neill said.

"The fact is, we can't afford to lose any of our squad at the moment, but besides that, Ji is a player who has done well for us.

"The lad has talent, but he's just come over here and is still learning the game. We want to see how he progresses and develops.

"Apart from that, he's just scored one of the most important goals that I can remember - that amazing last-minute winner against Man City and making himself an instant hero in the eyes of the fans - and myself - in the process."

O'Neill did not disclose which clubs had enquired about Ji, but the South Korean had reportedly attracted interest from Schalke and PSV Eindhoven before his move to Sunderland.