Due to these legal issues, Dong was loaned out to get first-team experience with Belgian First Division side Royal Antwerp where the employment laws were less strict.

At the beginning of the 2005–06 season, he was called up to join Manchester United on their pre-season tour and made his unofficial debut for the club in a friendly match in Hong Kong, scoring a goal in a 2–0 win.

In the 2005–06 season, Dong impressed for Royal Antwerp as he finished as the top goalscorer for both his team and the Belgian Second Division with 18 goals, including two hat-tricks.

With his eligibility confirmed, he was recalled from Royal Antwerp having started the season with 11 goals in 15 appearances, given a new contract until 2010,[5] and added to Manchester United's first-team squad on 17 January 2007.

Dong made his Old Trafford debut in a friendly charity match against a European XI on 13 March 2007, coming on as a substitute for Alan Smith in the 72nd minute of the 4–3 win.

He was a regular for the Manchester United reserves, and on 9 May 2007, Dong made his Premier League debut, partnering Ole Gunnar Solskjær up front against Chelsea at Stamford Bridge.

On 12 December 2007, Dong made his first UEFA Champions League appearance as a substitute against Roma in the last group stage match of the 2007–08 season, replacing fellow striker Wayne Rooney.

Despite impressing in several pre-season friendlies, Dong looked desperately out of his depth in the league and managed to play in just two appearances before being dropped to the reserves.

On the back of former teammate Cristiano Ronaldo's recommendation, Portuguese club Portimonense signed Dong to a one-year deal in August 2010.

Dong scored his first goal for the Chinese national team in a 4–1 loss to Switzerland on 3 June 2006 and was also called up for the 2007 AFC Asian Cup.