Alonso is currently considering his future with his three-year nightmare at McLaren set to end when his contract expires after this season.
The two-time world champion has been unable to compete at the front of the grid since re-joining the Woking-based team because of an inferior Honda power unit.
However, McLaren are in the process of extracting themselves from their deal with the Japanese manufacturer, a move that could convince Alonso to stay with the team.
And a report from Motorsport today that Sainz has agreed a deal to join Renault appears to strengthen the possibility that Alonso could remain at McLaren.
Renault are said to have been given the chance to sign Sainz as a sweetener as Toro Rosso try to terminate their own engine deal early.
Renault, who already supply engines for their works team and Red Bull as well, have already said they would be unable to add a fourth team.
But if Toro Rosso have managed to extricate themselves from the Renault power unit supply deal, it would mean they could instead supply McLaren.
And while Alonso has talked up the possibility of crossing the Atlantic to race IndyCar full-time after his foray into Indy500 earlier this year, his first port of call would be to stay in F1.
The McLaren chassis is though to be one of the best on the grid and with a better engine, Alonso could find himself competitive again, the only condition on which he says he will stay with his current team.
The Sainz-Renault-McLaren domino process could reportedly even start before the current season ends.
Renault are said to be so unhappy with Palmer’s performance that Sainz could be in their car before the start of the Malaysian Grand Prix on October 1.
That would likely see Pierre Gasly called up from the Super Formula Championship to replace him at Toro Rosso and join current driver Daniil Kyat.