Fernando Alonso has denied regretting his decision to quit McLaren and F1.
"We spent the second half of last year trying to understand the problems with the car and things are now starting to be worth it," Sainz said in Bahrain.
"We are learning, improving the car, and today we are here with the best, which was unthinkable three or four months ago," the Spaniard added.
Alonso struggled with McLaren with both Honda and Renault power between 2015 and 2018, and at the end of last year decided to quit.
But he was on the grid and pitwall in Bahrain, and this week he will even test for McLaren.
One might be forgiven for thinking the 37-year-old would prefer to be in the cockpit of the improving McLaren car.
"You would have to ask Fernando that," said sporting boss Gil de Ferran, a key Alonso ally, "but we are happy to have him with us at the test this week."
Alonso said in Bahrain that he is "happy" that McLaren's new drivers like the 2019 car.
But he told the Movistar broadcaster that he does not regret quitting.
"This year, the decision was to face other challenges," Alonso, who will contest the Indy 500, said in Bahrain.
"Last year I left here fourth in the championship. I was fifth in Australia and seventh in Bahrain.
"I have been here for 18 years, so I think to be ninth or tenth again, I'd rather not be here," he added.