Jenson Button believes Lewis Hamilton's impending exit from McLaren will be “a big loss” to the team.
Button has previously made it clear he has enjoyed pitting his wits against a fellow world champion in Hamilton these past three years, helping to make him a better driver.
Although there will be no second crown for either man this season, a 24-point gap between the British duo means Hamilton is set to leave for Mercedes with a 2-1 victory in their inter-team rivalry based on total points at the end of the season.
Rising star Sergio Perez may have been recruited as replacement, but the 32-year-old Button cannot help but feel Hamilton's departure will be felt by the team.
"With Lewis leaving, obviously the team loses a very fast driver," said Button.
"He's a guy that's achieved a lot with the team, wins and obviously a world championship - the last guy to win a world championship for the team.
"It is a big loss, but things change and you learn to move on and adapt. It's definitely not the end of two strong drivers at McLaren."
Although Perez has acquitted himself well at Sauber over the last two years, the 22-year-old Mexican still remains an unknown quantity.
Perez has conjured three superb podiums this season but his results have otherwise been sporadic, and notably since he was announced as Hamilton's replacement he has failed to score a point in three races.
Assessing Perez's potential, Button added: "I think Sergio will be fast, but I really don't know.
"He's obviously unknown in a McLaren because he's not there yet. But it's a great opportunity for Checo, and I'm sure he will take it with both hands."
Hamilton, meanwhile, insists he has no regrets about joining Mercedes, who are currently on their worst run since returning to the sport at the start of 2010.
Like Perez, since Hamilton announced he was moving to the Brackley-based team on a three-year contract, they have also failed to pick up a point in the last three races.
When you consider that in the previous 52 races Mercedes had not scored in only four, it highlights just how much they have plumbed the depths.
Asked by Press Association Sport about Mercedes' run, Hamilton said: "I've been told about it.
"It makes me even more driven to help them score points, and no, I'm not thinking 'have I made right decision?'
"I'm very happy with the decision I made. What is happening to them doesn't have any impact on that.
"People do sometimes lose their way. Even we (McLaren) have struggled in the past and not scored points."
Appreciating the size of the task awaiting him at Mercedes when he officially joins on January 1, Hamilton said: "I already knew how big it was going to be.
"I really thought about it long and hard, and I could see how big it was. Perhaps it's now growing.
"But as a task I'm really excited about it, excited to meet new people and to being in a different environment, and to mould myself into that environment."