Juan Pablo Montoya says it is frustrating to see someone of Fernando Alonso's calibre struggling at the wrong end of the grid.

The 2015 season has been a disastrous one for Alonso and McLaren as their new power unit partner Honda has failed to come to grips with the current engine regulations.

Instead of fighting for victories and podiums, the Spaniard and team-mate Jenson Button are now considered back markers during most races.

Alonso has retired in eight of the 16 races he has taken part in this season with his latest retirement in the Mexican GP coming after just one lap due to yet another engine problem.

Montoya admits he's not sure how Alonso is able to cope.

"Fernando is my hero, I am not sure how he manages to endure the current situation, I would not be able to handle it, it is very hard for a driver like him," the Colombian told Spain's AS.

The two-time World Champion has had several outbursts over the radio this year with his biggest one coming at Suzuka when he equated his Honda unit to a "GP2" engine and labelled the team's performance "embarrassing".

Montoya added: "I fully understand his frustrations, he is an extraordinary driver and what happened in Japan with those radio messages is logical, I would have done the same in that situation."

Asked for his opinion on the best drivers, Montoya replied: "I think [Sebastian] Vettel is the most complete and the hardest working, but for the driver's speed it is Fernando Alonso. [Lewis] Hamilton is between Vettel-Alonso, which is why he is doing very well."