Ronaldo, in his second spell at Old Trafford, had begun to look surplus to requirements and a move away in the transfer window looked likely at some stage.
But the Portuguese, handed a starting spot in Moldova after being restricted to the bench in the Premier League, struck a first-half spot kick to register his 699th club goal.
Ronaldo's first goal of the season sealed a Group E win that was essential after Manchester United had lost their opening game at home to Spaniards Real Sociedad.
Ten Hag said Ronaldo still had a big part to play.
"He's totally committed to this project, totally committed to this team, totally involved, he also has connections that are coming around him and he is constructing connections so I'm happy with that," Ten Hag said.
"He has to work really hard and invest to get the right fitness. He will score more goals. He's really close, when he gets more fitness he will score more."
United's win, their third consecutive away from home in all competitions, lifted them to three points in Group E, second behind Real Sociedad who have six.
United won't play until October now after their game with Leeds United this weekend as the game is postponed due to stretched security services in the United Kingdom because of the death of Queen Elizabeth II.