Michael Schumacher has declared that Norbert Haug's departure from Mercedes will "tear a massive hole" in both the German firm's motorsport efforts and Formula 1 itself.
Haug is leaving his role as Mercedes' motorsport chief after 21 years in the position.
His tenure included Schumacher's first big break in Mercedes' World Sportscar Championship team, and the seven-time world champion's F1 comeback with Mercedes' revived factory F1 programme in 2010.
"Since I entered professional motorsport, Mercedes and Norbert Haug together were part of it, so this step will mark a big break," Schumacher wrote on his personal website.
"We spend a lot of years together, being sporting combatants or allies, and Norbert has always been into this with full enthusiasm and wholeheartedly.
"He was living motorsports, and him leaving will tear a massive hole in both our sport and our team."
McLaren boss Martin Whitmarsh, whose team was Mercedes' works F1 squad from 1995 to 2009 and still uses its engines, declared earlier on Friday that Haug had been a "crucial" factor in McLaren's success.