The team's Lando Norris qualified second for last week's Dutch Grand Prix behind Max Verstappen, the latest evidence of the giant leap McLaren has made with its B-spec upgrade it deployed before the summer break.
That overhaul allowed it to challenge Mercedes, Ferrari and Aston Martin in the ever-fluctuating fight behind the Red Bulls, with no team being able to consistently claim the status of second-fastest team.
But team principal Stella cautioned that the difficult mixed weather conditions across the Zandvoort weekend flattered McLaren. While it introduced a new rear wing to good effect, Stella thinks the team will need regular development to be second on merit.
"To be able to do that, we need to keep developing the car," Stella commented on McLaren's one-lap speed.
"Because I think, at this track, in normal, dry conditions potentially Mercedes could have been there with us. They were faster than us in Hungary. If you remember they had pole position, actually.
"So, I think the current package is not enough to be able to fight with Red Bull, certainly, but it's also not enough to be going to a race and say we're going to be the second fastest here.
"We need to introduce further improvements, which is what we plan to do for the next couple of races."
On race pace Mercedes also appeared superior to McLaren in the Netherlands, with Lewis Hamilton particularly impressing with his lap times.
Hamilton came from behind to lead Norris home in sixth, though both teams were on the wrong side of an early strategy call by staying out on slicks in the wet and wasted any podium chances.
Addressing the MCL60's lack of aerodynamic efficiency remains a priority, especially at a low-downforce circuit like Monza where it will introduce a new low-drag wing.
But Stella explained that another key development area for the Woking squad is making the car generate downforce more consistently across various circuits, corner types and conditions, which has arguably been the dominant Red Bull RB19's greatest virtue.
"While we are addressing the overall efficiency, we are also trying to make sure that the downforce is less variable as a function of ride heights, yaw angles and so on, which I think is what affects cars that have good peak downforce, but then this downforce reduces rapidly as a function of these attitudes," he added.
"So, I think we have improved compared to the start of the season, but we are halfway in the journey compared to where we think we should be, to be at the top of the grid."
