Aston Martin's technical boss is waiting for the details of the three-race 'super qualifying' trial for 2021 before giving it the thumbs up.
The F1 teams have given a tentative green light to the proposal that, at three race weekends in 2021, a 'sprint race' on Saturday will set the grid for Sunday's main event.
But Green is quoted by Italy's Autosprint as revealing that the teams still don't know the details of the plan.
"We need a set of rules and we still don't have them," he said.
"We have seen a proposal that I think most of the teams have expressed a favourable view about, but the devil is always in the details and we don't have them yet."
Green said the biggest problem is that Formula 1 and all of its other rules were designed around the idea that the only actual racing takes place on a Sunday.
"Will we be allowed to change the brakes? More importantly, what happens to the allocation of power units?" he wondered.
"In some ways we're stuck because the engines were designed and tested for a certain type of season, and moving away from that perspective will be quite challenging.
"So there is a lot to discuss, a lot of details to work on, but I think there is a general willingness to try it. However, as I mentioned, the devil always hides in the detail," said Green.
