Verstappen was chosen over Sainz for a Red Bull promotion in 2016, with the Spaniard going on to drive for Renault and McLaren, where he looks well-placed to secure 'best of the rest' status behind the big three teams' drivers, if not even sixth place, with Alexander Albon now at Red Bull in place of Pierre Gasly.
The Dutchman's faint remaining title hopes in 2019 were likely extinguished in Belgium as he crashed out on lap one, with Hamilton opening up his championship lead over team-mate Valtteri Bottas to 65 points by finishing second behind Leclerc in Spa.
However, Sainz believes Verstappen is the closest competitor to Hamilton in terms of ability, with the championship leader freed up to perform better in recent seasons after former rival Nico Rosberg's retirement.
Asked by Movistar to rank his contemporaries, Sainz said: "Hamilton: nine or 10. Since Rosberg left, he's been at a very high level, with less pressure.
"Verstappen? Almost better than Hamilton, he's a 'hit,' but in Spa he was wrong.
"Bottas? He's always there, but he doesn't finish it off. He's a driver at a very high level, but Hamilton has something difficult to explain. It seems he is relaxed, smoking a cigar, and is always there."
Turning attention to Ferrari, Sainz was largely unmoved by Leclerc's run of six straight qualifying victories over Vettel.
However, the Spaniard sees more cause for concern that Leclerc has outscored the four-time world champion by 13 points since his home DNF in Monaco.
"If I were Vettel I would worry about Sundays with Leclerc," Sainz said.
"On Saturdays, a young man can beat you, but if Leclerc wins on Sundays that pace should worry him."
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