Dan Towriss, chief executive of Formula 1's newest team and one backed by General Motors, told the 'Off Track with Hinch and Rossi' podcast that Herta, 25, was leaving IndyCar to pursue his F1 dream.
Formula 2 is the feeder series for Formula 1 and offers the chance for Herta to secure the super-licence points he needs.
It remains a junior series, however, unlike IndyCar, whose signature Indianapolis 500 ranks among the world's great challenges, and Herta will be leaving a top seat for an uncertain future.
"We’ve been fortunate to keep Colton at Andretti in the IndyCar team, and so now he’s going to pursue his dream in Formula 1," said Towriss. "And to do that, he’s going to take a pretty big risk.
"He’s leaving IndyCar, he’s not going directly to Formula 1. There is a test and development driver role and he’s going to be going into F2. He’s got to learn tracks, he’s got to learn tyres...
"I couldn’t be more proud of Colton, to be willing to take that risk to pursue his dream."
Cadillac announced Herta's new role in a statement on Wednesday. The team have already signed F1 veterans Valtteri Bottas and Sergio Perez as next year's lineup.
F1 drivers at Monza, which hosts Sunday's Italian Grand Prix, said the American was already well-equipped to race against them.
"IndyCar I think is one of the toughest series in the world," McLaren's Lando Norris, who came through F2, told reporters.
"I think it's an incredibly tough car to drive... the level of all those drivers is incredibly high."
Norris suggested IndyCar drivers should score more licence points than they are currently awarded for their results in the championship.