Sainz could slot in at Red Bull but the previous pairing of him and Verstappen at Toro Rosso was far from smooth and the Dutch 20-year-old is now the senior team's clear focus.
"It is the toughest decision I have made in my racing career".
At Renault, though, he is more likely become the star figure ahead of teammate Nico Hulkenberg and therefore, that may have swayed Ricciardo's decision.
There's also an assessment of the knock-on effects that may follow Ricciardo's move, including who will take his place at Red Bull and what the development means for Esteban Ocon.
Ricciardo is now fifth in the points standings, 14 points behind Valtteri Bottas in fourth and 95 points behind Hamilton in the points lead.Ricciardo has won seven races in his F1 career, including three in his first season with Red Bull Racing in 2014.
Can Ricciardo return Renault to the top of the sport? Renault finished ninth in 2016 with Jolyon Palmer and Kevin Magnussen in the paddock but upped their game in 2017 with Nico Hulkenberg scoring the majority of the team's points as they finished sixth.
Red Bull are ditching Renault power for Honda next year after a raft of reliability failures.
Ricciardo is relatively low-key about the Renault move, even though it is just as big - or bigger - than his original promotion to Red Bull for the 2014 season alongside four-time champion Sebastian Vettel. Ricciardo added: "I hope to be able to help them in this journey".
A part-season with HRT paved way for his full-time F1 graduation with Toro Rosso in 2012, and he was then brought into Red Bull's main team to replace Mark Webber, winning seven races for the squad since '14.
Red Bull meanwhile need to find a replacement for Ricciardo.
Until Red Bull decide, Sainz's hands are tied and any other move - and McLaren have expressed interest - must remain on hold.
The unexpectedly open seat at Red Bull is now one of the most appealing targets on the F1 grid for next season.
"I really appreciate it, from the bottom of my heart, and we've got nine races left to kick some arse, so let's get it".