It could take McLaren "years" to get back to the front of the grid.
That is the warning of former team driver Jenson Button, as he was asked about Carlos Sainz's move from Renault to replace the retiring Fernando Alonso.
'They've fallen too far behind'
"I don't think the pace of the car will change in 2019.
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They've fallen too far behind. It's a great team that has won many world championships so they can do it, but it will take time," Button told El Mundo Deportivo newspaper.
Button said 23-year-old Spaniard Sainz will therefore need to be patient as he pushes for wins and titles.
"He will need to understand that it will take a few years of hard work. He will need to make himself heard and his opinion count, and hopefully bring the car forward.
Filling the Alonso voide
It will always be difficult to replace Fernando, but they wouldn't have chosen him if they didn't believe he deserved it," Button added.
Another former Formula 1 driver, Marc Surer, thinks McLaren went off the wrong track during the Honda years.
"If you look at where they are now, I think Honda was never so bad.
They'd be exactly where they are if the Honda was still in there. I think the engineers were fooling themselves by thinking they had the best chassis on the grid.
They were slowest on the straights and fastest in the corners, but anyone can do that if you have enough downforce on it," Surer added.