Motor racing legend Mario Andretti has said that Fernando Alonso is "wasting" his time at McLaren-Honda.

Last weekend, the Spaniard skipped Monaco in order to tackle the Indy 500 with the team run by Andretti's son and former McLaren driver Michael.

Alonso, 35, will be back in his McLaren-Honda in Canada, even though 1978 world champion Andretti has advised him to consider a full-time Indycar foray.

"The only thing that keeps a driver happy is to be competitive," the 77-year-old is quoted as saying by Spain's Marca.

"Anything else is total frustration. He is in the best phase of his career and wasting his time. McLaren obviously feels the same pain because it's a team that only knows how to be with the best."

Andretti does think that there are better times ahead for both McLaren and Alonso.

"It can only go better for him," he said. "It looks like they have a [good] chassis, and the situation with the engine will improve. I'm sure he'll be in the points before the season is over."

He also said that Alonso was right to sit out Monaco in order to try to win at Indianapolis, even though it was a familiar Honda engine failure that let him down.

"I guarantee that his value has gone up," said Andretti, "and he deserves the respect he gets. There were no losers here, except for the engine."

McLaren remain rock-bottom of the constructors' standings heading into the Canadian Grand Prix.