Luis Diaz put Liverpool ahead in the first half at Old Trafford before Bruno Fernandes equalized and Mainoo made it 2-1 for United in the second period. Mohamed Salah's late penalty rescued a draw for the visitors.
Defender Kambwala made his second league start of the season on Sunday with Raphael Varane and Jonny Evans out due to injury.
"I don't think, at this short notice, they will return," Ten Hag told reporters of Evans and Varane.
"We were convinced (Kambwala) could do the job and he has done brilliant."
He was delighted with the contributions of Kambwala and Mainoo.
"I am very pleased, very happy and it is another signal and message of the future of Manchester United.
"This team, the squad has high potential and if we get more players available, especially in the backline... then the future will be very good."
United are sixth in the Premier League and look set to miss out on a place in next season's Champions League.
They visit Bournemouth on Saturday before playing Coventry City in the FA Cup semi-finals on April 21st.
"It's still positive, we have the semi-final," said Ten Hag.
"We will keep fighting for every point and see where we are in the end."
