SINGAPORE (Reuters) - Mark Webber could have stayed in Formula One and challenged for grand prix victories for several more years had he not "fallen out of love" with the sport, according to Red Bull team principal Christian Horner.
Australian Webber, 37, announced in June he was leaving the world champions, winners of the last three constructors' titles, at the end of the season to join Porsche in their Le Mans sportscar programme.
"Mark's chosen to leave and only he knows the full reasons behind that," Horner told Reuters ahead of Sunday's Singapore Grand Prix.
"In terms of his performance and capability, I think he could have done another couple of years, but I think he's fallen out of love a little with Formula One."
Webber, winner of nine races in seven seasons with Red Bull, has been eclipsed by triple champion team mate Sebastian Vettel since the German took his first title in 2010 and their relationship has been frosty.
The Australian will be replaced by compatriot Daniel Ricciardo next year.
"He (Webber) has decided to take on a new challenge in sportscar racing and he has always had a passion for that kind of racing, that's where he came from to Formula One, and he's chosen to pursue his career in that direction," said Horner.
"Could he have stayed in Formula One for another year or two? He is fit enough, he is quick enough, I would have thought he could have."
Webber's fractured relationship with Vettel may have played a part in his decision to leave Formula One's top side, but the Australian has denied this. He has also been critical of the way the racing has changed to put more of a premium on tyre management.
The Red Bull pair had a public spat in March at the Malaysian race when Vettel ignored team orders to pass Webber and take the chequered flag leading to a row before the podium presentation.
While Vettel is heading for a fourth successive title with a healthy 53 point lead, Webber is fifth in the standings and 92 points behind his team mate with seven races remaining.
Horner said Ricciardo, 24, would be able to challenge Vettel on equal terms.
"The key factors behind the signing of Daniel Ricciardo for next season were fairly simple, one he is tremendously quick, two he is a product of the Red Bull junior programme so we know him very well and three he is very young and the best guy to fill the shoes that Mark Webber has left behind.
"Our expectation will be for him to push Sebastian Vettel as hard as he possibly can. For sure it is going to take a couple of races for him to get up to speed but... I think he will settle in very quickly."
With a raft of new rule changes for 2014 Horner was unsure if Red Bulls' dominance would continue.
"It's a clean sheet of paper for everyone, and we've got a very talented technical group, they've demonstrated that with the last big regulation change, and who knows who's going to have the best power unit next year," he said.
"Renault, Mercedes, Ferrari...Honda coming in 2015, but until we turn up in Melbourne for the first Grand Prix it's going to be very difficult to predict." (Writing by Patrick Johnston, editing by Alan Baldwin)