Fernando Alonso was bad for the Ferrari Formula 1 team because he was too egocentric and critical, according to three-time world champion Niki Lauda.
The Spanish driver left Ferrari at the end of last year after five seasons during which he finished runner-up three times before enduring his worst campaign with the team in 2014, when he was sixth in the standings with just two podiums.
Alonso has returned to McLaren for 2015, but has struggled at the back of the field so far as engine supplier Honda gets to grips with the power unit.
Meanwhile, Ferrari has taken a significant step forward compared to 2014, with Sebastian Vettel winning the Malaysian Grand Prix as the Maranello squad has emerged as Mercedes' closest challenger.
Lauda, a non-executive chairman at the Mercedes team, says Vettel is exactly what Ferrari needed, labeling Alonso as "dark".
"Alonso is egocentric, dark and grumpy," Lauda told the Repubblica newspaper.
"He's also quite negative. Last year right after the races finished he started criticising: this doesn't work, there's a problem there, this is why we are not winning.
"How can you keep the morale of a team up if the driver just says bad things? Everybody knows it: if you are racing for Italy that's an attitude you can't afford.
"Maybe somewhere else they can ignore the criticism, but not at Maranello and its surroundings. Vettel is sunny, Alonso is dark.
"He made the wrong decision. He pushed himself into the abyss."