The double world champion did not reject his return by confessing him unbelievable at 37, but team boss Zack Brown said on Wednesday that the Spaniard would be the first driver he had called if there was a problem.
"We will certainly not hesitate if we needed a driver who is not one of our current two to have Fernando first on the fast-track list," he said.
"(Michael) Schumacher returned, (Alain) Simple came back and I did not want to explain it because we are very pleased with our driving for the future we are very focused on at this point never say never.
"He's certainly on top of his game."
Jenson Button, the 2009 world champion who left McLaren in late 2016, made a one-time return to the Monaco Grand Prix next year when Alonso was absent in the Indianapolis 500 race.
The Belgian Stoffel Vandoorne made his debut in 2016 when Alonso was injured in Australia and dropped out of race two in Bahrain.
McLaren has a whole new addition for 2019, with Spaniard Carlos Sainz joining Renault and British teenager Lando Norris, stepping up their reserve role.
Monaco winner Alonso will continue the Toyota endurance world championship after winning Le Mans 24 Hours this year and again with McLaren in Indy 500 while pursuing a "triple crown of motorcycle".
Brown said Alonso is sure to be present at Formula One competitions next year and ultimately does not rule out a new role for him.
"He talks about one of these days as a shirt manager," said the American. "He likes to be on racing tracks: when he's not on track or endurance on Formula One, you'll find him on the golf kart track and he can not stay away from racing tracks."
"He's still a McLaren driver, we do the Indianapolis 500 together, so I think Fernando has other ways to influence and help our team."
Braun welcomed the Spaniard as one of his greatest eras and one who could win many more champions than he and Renault.
The Sunday race at Yas Marina, he said, will be emotional if it's also a disappointment.
"It is a shame that he will not have a chance to win or podium – this is probably disappointing for all of us because he runs the best he ever ruled," said the American.
"This weekend I'm sure it will be difficult even to get into the points."
Former champions of McLaren are sixth in the ranking of 62-point designers, 50 of which are marked by Alonso, who last won the Ferrari Grand Prix in 2013 and did not have a podium in 2014.
"Unfortunately, thanks to his long career, he won only two championships. If he was in the right cars during this 17-year career, he could be in the same place as Luis (Hamilton) and Michael (Schumacher) with five, six, seven, said Brown.
Alonso would surely have won a third title in 2007 after McLaren had favored overmate Hamilton with whom the season ended, while Kimi Raikkonen of Ferrari scored another.
Hamilton then won the title in 2008, while Alonso returned to Renault before joining Ferrari in 2010, the year he led the championship in the final Abu Dhabi race before the strategy broke his hopes.
The return to McLaren, unthinkable after the bitterness in 2007, came in 2015, but also coincided with the worst form of the team and a highly devoted partnership with Honda, which ended last year.
TALENT STATISTICS FULFILL
Ferrari also did not win the drivers' title from Raikkonen.
"His statistics are probably insufficient for his tremendous talent," Brown said, rejecting the conversation about a Spaniard as a devastating influence and emphasizing his intensity and hard work behind the scenes.
"I have been aware of the stories over the years, but from my point of view, he is a fast driver of racing cars who wants the best equipment and the best of the people and accepts nothing but perfection.
– It was a pleasure to work with him.