Bruno Fernandes was sent off for the second consecutive game, while it was substitute Harry Maguire who headed home on 91 minutes to earn the Europa League point for the visitors.
United had jumped to a 2-0 lead through Marcus Rashford and Rasmus Hojlund, before Porto fought back via goals from Pepe and Spain U21 striker Samu Omorodion.
"We will get there," Ten Hag told TNT Sports. "Don't judge us in this moment, judge us at the end of the season.
"We are in the process, we will improve. We have had two seasons where we have reached finals, we will continue and fight.
"You see the spirit and the connection between the staff and the team.
"The players are together, they have a strong spirit and they want to achieve. We have a good mentality but in some defending parts we have to step up.
"We had three clean sheets not a long time ago so we can defend very good, but we have to go back to those habits."