However, Alonso defended the Belgian.
"I don't think it should damage his reputation," said the Spaniard.
"It was the same at Silverstone - there is something wrong with his car."
Nevertheless, the Sainz rumours would that mean McLaren could line up in 2019 with an all-Spanish driver lineup.
"Carlos Sainz is a great driver and we could be interested in him," McLaren boss Zak Brown admitted.
Asked if the all-Spanish lineup would be an issue, he answered: "From the commercial point of view, it would be good for us.
"For the sponsors, the important thing is that we have the best two drivers available."
Sainz, though, was not commenting on the McLaren rumours.
"I never listen to rumours," he told Marca.
"But I am used to talking about my future at this time of the year and I tell you that I want to sign a contract of at least two years."