Some in Spain might consider it a dream team, but former Formula 1 driver De la Rosa is not so sure.
"A dream? I think it would be better if they were not together," he told Cadena SER.
"Being Fernando's teammate is very difficult, so I prefer that someone other than Carlos suffers. I would prefer if Carlos gathers two or three more years of experience and then face a beast like Fernando."
However, he said that if Sainz is ousted by Renault, the Spaniard should not worry about being spat out by F1.
"If Renault makes the mistake of letting Carlos go, he has still earned his place in F1," De la Rosa said.
More broadly, the former McLaren driver said that the fight for the world championship will still be closely fought, even though Lewis Hamilton is running away with the points lead.
"Mercedes is winning where Ferrari should, so the great drama for [Sebastian] Vettel is that despite having the best car he has 24 points less," said De la Rosa.
"There's still a long way to go and everything will be decided over details at the last race."
Sainz has picked up 30 points for Renault so far in 2018.