Asked why Renault simply didn't add a fourth team to its F1 roster rather than losing Toro Rosso, Renault F1 president Jerome Stoll said at the Frankfurt Motor Show: "Technically, we cannot.
"Otherwise, it would be to the detriment of one of the three teams we supply," he is quoted in a French-language AFP report.
"After all we are not a simple supplier in F1, but an actual actor of the competition. Our problem is the strategy we have put in place to reach the podium by 2020."
It is widely believed that, tied up with the complex deal, Toro Rosso owner Red Bull has agreed to release Carlos Sainz to the works Renault team.
That is despite the fact that some see Frenchman Esteban Ocon as a more natural teammate for German Nico Hulkenberg at Renault.
Asked about that, Stoll said: "We talk to everyone.
"You have to see who is not free, who is free completely, or who is partially free with an elastic to go back. With Esteban, he has a contract with Mercedes," he explained.
"So if he comes, it is for a given period. So is it interesting to have him for this period and then, when it could get even better, that elastic is pulled? It may make sense or it may not."
One interpretation is that the driver market will become even more interesting after 2018, when the deals of many drivers expire.
But Stoll said: "In fact, there will not be so many places, since Vettel and Hamilton have re-signed. So those who target Mercedes and Ferrari are likely to be disappointed.
"We are a very good option for very good drivers," he added.