Park has made over 160 appearances for United since signing in 2005
Manchester United midfielder Park Ji-sung has been ruled out for four weeks with a hamstring injury.The 29-year-old only resumed training last week after a month away with South Korea at the Asian Cup in Qatar.
"We got a blow on Saturday with the news Ji-sung Park did his hamstring on Friday afternoon in training," United manager Sir Alex Ferguson told ManUtd.com.
"He'll be out for up to a month, which is a blow to us."
Park is set to miss Saturday's visit of Crawley in the FA Cup, the trip to Marseille for the first leg of their Champions League last-16 tie and Premier League matches against Wigan Athletic, Chelsea and Liverpool.
"It was very unfortunate because it was his last kick of the ball in training as well," stated Ferguson.
"We were really looking forward to having him back after being away for a month at the Asian Cup."
There was, however, better news for United with regards to fellow midfielder Antonio Valencia, who started training with the club's reserves after a five-month lay-off.
The Ecuador international suffered a broken ankle and ligament damage in an innocuous challenge during United's 0-0 Champions League draw with Rangers in September.
"The good news is that Valencia started training with the reserves on Saturday," Ferguson added. "So he's on his way back and that will be a real boost to have him available for the run-in."
It was initially feared Valencia would miss the entire season, but after undergoing an operation and continuing with a rehabilitation programme the indication is that he could play again in March.