The two old rivals played out a 2-2 draw at Old Trafford as mangerless Leeds failed to hold onto a two-goal lead.
Wilfried Gnonto's first minute strike and Raphael Varane's own goal shortly after half time were cancelled out by Marcus Rashford's 20th goal of the season and substitute Sancho's strike.
It was only Sancho's second game for United after the World Cup as he struggled with his fitness.
But Ten Hag was pleased he could call upon the former Borussia Dortmund forward when the going got tough.
"I am pleased and proud he did it," said Ten Hag after the game.
"It is clear it is a difficult period but he finds himself out and he needs credit for that. He is a brilliant footballer and if he can invest in the right levels he can be outstanding.
"I will back him, the coaches will and the team will but finally he has to do it himself."
"It is in his own hands. If he wants, he can do," said the United boss.
"We now have a team that can play in the opponents' half. He likes that. He can play in tight areas.
"At this moment he is in a good mood and it will strengthen him, it will motivate him to give more.
"He is in the right direction and if he keeps going with that process he can do even more to really help us to achieve our goals."
The Red Devils will face Leeds United again on Sunday at Elland Road.
