The 24-year-old had drawn criticism from L'Equipe TV presenter France Pierron for leaving the squad mid-tournament to attend the birth of his son, Praise, who was born on Monday.
Doku featured in Belgium's opening 1-1 draw against Egypt but missed their goalless draw with Iran due to breathing issues in training.
"Jeremy came back three days ago. So, he only participated in the practice today. He did a little more, but he did not train during seven days and that's not insignificant," Garcia told reporters.
"So, we're thrilled to have him back. He's probably not going to play for 90 minutes. But we know that he can make the difference once on the pitch.
"But all our game is not based upon one player. It's the collective spirit. That's the true force of Belgium."
Striker Romelu Lukaku, still searching for his first goal of the tournament, is also unable to play a full match.
"Lukaku cannot handle 90 minutes," Garcia added.
"If he starts, he will be substituted. If he doesn't start, he can help us from the bench. But he's fit."
Belgium centre back Brandon Mechele said the players were pleased for Doku.
"It's lovely news that he became a father and that he was also present with his wife to help her. So, everybody was happy. It gives a boost for him, but also for us that he's back, that he can help the team," Mechele said.
Despite being widely tipped to top the group, Belgium head into Friday's match in third place with two points.
Egypt lead with four, while Iran also have two points but are above Belgium on goals scored. New Zealand, still seeking a first World Cup victory, are bottom with one point.
"We need to match their intensity. If we manage that, I don't see a problem for us," defender Mechele added.
"On paper, we have more quality than New Zealand with all due respect to them.
"We might have lacked that intensity a bit in the previous match, but that was a wake-up call. I think we will be ready tomorrow."