The Spaniard has this weekend put one hand firmly on the Formula Renault 3.5 title by winning both races at Paul Ricard. The double triumph puts him 44 points clear of rival Roberto Mehri with just 50 left to score.
Most observers had assumed that Sainz’s success in the series - seen as the equal of official F1 feeder category GP2 - would guarantee him a spot at Toro Rosso for next term. However, the Red Bull-backed squad elected to hand 16-year-old Verstappen the chance instead.
And speaking to Crash.net, the Spaniard admitted he had not expected to be leapfrogged by the Dutchman.
“It was a shock,” said Sainz at the French circuit. “I just need to focus on myself and win the championship to prove to all the people who think I am not good enough, that I can be the first Red Bull junior to win the [FR3.5] championship and youngest ever too.
“It was a big surprise for me," he continued. "It was unexpected, especially when you are leading a championship, but these things happen. I would not be in this championship if was not for Red Bull so I am thankful for that. It is a tricky situation.”
Red Bull have stated that Sainz still has a future with their programme, with a race seat elsewhere in F1 or another series a possibility for 2015.
“I had a meeting with them and they want to keep helping me in the future and they want me to still be part of the Red Bull family," Sainz confirmed earlier this month.
"This meant that they think I have something special still. They want to keep me.
"I don't know in which position or which role, but it gives me a lot of confidence that they haven't kicked me out.”
Sainz has been linked with a spot at Caterham during the final few races of 2014, though FR3.5 rival Merhi is also in the frame having run FP1 for the squad at Monza.