Ten Hag unsure when Sancho will return to Man Utd first team
Sancho, who was not called up for England's World Cup campaign in Qatar, has been training separately from the United squad in recent weeks and did not feature in their friendly games against Spanish sides Cadiz and Real Betis.
He also missed United's last four matches due to illness before the World Cup break.
"He's not fit enough to be here," Ten Hag told British media at United's training camp in Spain.
"First we didn't see him the last games for United. So he wasn't in the right status, in the right fitness state, so now he's on an individual programme and we want him to finish that programme and hopefully, we see him back soon.
"We want to bring him back as quickly as possible but I can't give a prognosis of when that will be."
Sancho began the season well, scoring in league wins over Liverpool and Leicester City, but the winger has scored no goals and recorded just one assist since mid-September.
Ten Hag said Sancho's issues were down to "a combination of physical but also mental".
United return to action on December 21st with a home League Cup tie against Burnley before hosting Nottingham Forest in the Premier League on December 27th.