Sir Alex Ferguson admits it is disappointing that Manchester United will lose Park Ji-sung for several weeks after Christmas.
Park again proved his worth to United in big matches by scoring the only goal in Monday's home win over title rivals Arsenal.
The improvised header was Park's sixth goal of the season and it is the South Korean's best return since joining United from PSV Eindhoven in 2005.
However, the midfielder is due to represent Korea in the Asian Cup in Qatar in January and the Boxing Day game with Sunderland will be his last before linking up with his country.
"Ji is in fantastic form," said Ferguson. "Unfortunately we are going to lose him for seven matches after December 26. It is disappointing."
Park commented: "I have mixed feelings about the Asian Cup.
"At the moment I am playing with confidence, so I do feel a bit strange about going. But I am always happy to join my national team as well.
"Even without me Manchester United have enough players to keep winning, so I will concentrate on the national team and then focus on Manchester United."
The 29-year-old's goal lifted United back to the top of the table and Park has urged his team-mates to maintain their current form.
He continued: "At the moment, we are in a great position in the league. Now we have to concentrate on the games coming up.
"We always start to play well at this time of year and want to keep this going.
"We played with great confidence against Arsenal and our defence was unbelievable."