Nico Rosberg has accused Lewis Hamilton of 'losing his way around Monaco' after watching the seven-time F1 champion struggle during Friday practice ahead of the grand prix.
Hamilton and Russell suffered across the two hours in a car described by team boss Toto Wolff as "undriveable" following the first session around the principality's streets.
With the car running stiff as part of this year's package, it was unable to cope with the bumpiness of the principality's public roads, leaving Hamilton to declare his eyeballs were coming out of their sockets.
Hamilton finished a lowly 12th on the timesheet, 1.6secs behind pacesetter Charles Leclerc in his Ferrari and almost a second down on team-mate George Russell.
Assessing what he was witnessing from his former team-mate, speaking to Sky Sports F1, Rosberg said: "This weekend, Lewis has been having a hard time out there.
"We know the Mercedes is difficult to drive but George has stuck with it and found that flow. He is making incredible progress and is in that fight with Lando for best of the rest after Ferrari and Red Bull.
"Lewis has just lost his way out there, so it is going to be very tough for him to come back and find that rhythm with only a couple of laps left in FP3. It is a big setback for him."
Ricciardo crash 'exactly what he didn't need'
McLaren's Daniel Ricciardo has also been enduring a hard time this year, with the Australian suffering further misery with a heavy crash into a barrier in FP2 that left him bottom of the standings without a time to his name.
In stark contrast, team-mate Lando Norris finished only behind the two Ferraris and two Red Bulls despite still recovering from tonsillitis that left him struggling across the recent Spanish Grand Prix weekend.
"It's exactly what Daniel didn't need," added Rosberg. "Here, he knows this track so well, it could have been a great weekend for him and again, he's got this big setback.
"It's so difficult. He broke the rhythm with that and he only has a couple of laps in FP3 [to find it again].
"And at the same time, Norris is flying again this weekend. He is the best of the rest out there."