The Ferrari-powered Haas driver said that the works team of McLaren's new engine supplier - Renault - has "developed the project well and is already in front of us".
The Frenchman therefore expects McLaren to be strong with Renault power in 2018.
"McLaren will be very strong," Grosjean is quoted as saying by Italy's Autosprint.
"They have huge resources and probably the biggest structure in Formula 1.
"The Renault is a respectable engine - they've had reliability problems but in terms of power they are there now.
"The bigger question is about Williams, Force India, us and Toro Rosso with their new power unit. I think McLaren will be there with Red Bull, then there will be Renault."
McLaren ended the 2017 championship with 30 points.