Alonso is skipping the Monaco Grand Prix so he can compete in IndyCar's biggest event, with his place at McLaren being taken by Jenson Button.

But although many within the sport have welcomed the idea of Alonso racing in America, Horner thinks that Brown needs to see a "psychiatrist" for letting his driver compete at Indy.

"It is a difficult one for Fernando as he is having a tough time," explained Horner in Bahrain on Friday. "Zak [Brown, McLaren executive director] has this problem. He has a depressed driver on his hands and he is trying to keep him motivated.

"He has come up with this idea to send him to Indianapolis, but he must be barking mad. It is the nuttiest race I have ever seen.

"[There is] no testing. He is just going to jump in the car. Turn 1 is a proper Turn as well, it is not easy flat all the way around. So I think he needs to see a psychiatrist personally."

Not right thing

Horner added that he would never allow his team's drivers to do what McLaren have done.

"Would we let out drivers do it? No. I believe if a driver commits to a team, it is a bit like disappearing with another girlfriend halfway through the year and then coming back. It doesn't seem the right thing to be doing.

"Perhaps if the races didn't clash, or do it at the end of his F1 career. But obviously McLaren have got this approach which is perhaps different to ours, but good for them."

Williams deputy team principal Claire Williams also said that she would not have allowed her team's drivers to skip a race to compete in another race.

"I don't know whether that is because I am my father's daughter and I know Frank probably wouldn't," she said. "But no, I don't think I would. But I won't expand on reasons why."