The newest outfit on the grid made the improvement to finish P5, ahead of McLaren, while Force India would have been above the Woking team had it not been for the administration and exclusion issues.
“If they are actually now the strongest midfield team, I think we should all fear them,” Sainz said.
“But next year we should fear everyone. You think about Sauber, but then you think about Force India with a bit more budget, what they could be capable of.
“You think about Renault, what they are able to do. You think about Haas, if they keep getting the amount of Ferrari parts that they are getting and Ferrari keeps dominating [engine development], they should be also very strong.”
The Spaniard moves to McLaren with the team in their longest barren spell. It has now been six years since the team last scored a victory.
Sainz knows that it will be a struggle for the team next season, and that it could be anyone out front in the ‘Formula 1.5’ championship.
“You think about McLaren with a whole new project and wanting to invest and wanting to go back to P4 in the championship after a difficult year,” he added.
“You think about all these teams, and you think anyone could lead the midfield battle next year.”
The Spaniard’s main hope for the 2019 campaign is that the top three are brought back towards the midfield runners, though.
“What I really hope is that the space between midfield and the top teams gets a bit more close, and we have a bit more exciting racing,” he continued.
“Then who comes out on top, we will see. It’s about the one who gets the new regs better.”
The 24-year-old is also hoping that his former employers will provide his new employers with an engine that can challenge their rivals.
“I need Renault next year, for them to think about the engine and really make a step,” Sainz admitted.
“If not, Mercedes and Ferrari, even Honda, they’re going to keep evolving and creating this gap which is very difficult to cut back.
“It’s difficult to say, but I really hope this situation starts getting a bit better and we have a closer field.”