Manchester United forward Jadon Sancho. AFP
Sancho has not played for United since their 1-1 draw at Chelsea in the Premier League on October 22.
The 22-year-old was left out of England's World Cup squad and also missed United's training camp in Spain last week.
Sancho has found it difficult to produce for United on a regular basis since his £73 million ($90 million) move from Borussia Dortmund in 2021.
He also endured racist abuse after missing a penalty during England's European Championship final shoot-out defeat against Italy last year.
United manager Ten Hag said he had seen a further dip in Sancho's confidence after he was axed from the England squad for the Nations League matches in September.
"Sometimes there are circumstances with fitness and mood," Ten Hag told reporters. "He was not in the right status or fitness state."
Sancho scored three times in his first eight appearances this season, but missed United's final four games before the World Cup break and deleted his social media presence after being snubbed by England.
Ten Hag said Sancho has been working on his own fitness programme, using a coach in the Netherlands who has previously worked with the Dutchman.
"I have had several talks with Jadon. He's on a physical programme and our aim is to get him back as quickly as possible but I can't give a prognosis of when that will be," Ten Hag said.
United return to action after the World Cup with a League Cup last 16 tie against Burnley on December 21.
Ten Hag admitted Sancho might not feature immediately because he still has much to work on.
"When the league started he played some good games but after we got a drop of levels. Sometimes you don't know why or what is causing it," he said.
"Most of the time it comes slowly. First you observe but the stats back it up. In the start of the season he had goals and assists but his key moments and key actions became less and less.
"It's a combination of physical but also mental. We're trying to research and get him back."
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)