It was against Girona when the alarm bells started ringing. Real Madrid went a goal down at Montilivi in August and the home side threatened to score every time they attacked down the right side.
Julen Lopetegui’s side equalised before the break and then went ahead, but still they were vulnerable and the Basque made a decision. He took off Marcelo and sent on Raphael Varane.
There was only an hour on the clock and Marcelo was surprised. Afterwards, he said: "I respect the decision, but I don’t understand it."
But everyone else did. The Br azilian was not right then and he is not now. Lopetegui, of course, is long gone. He made many mistakes in his troublesome tenure, yet his judgement on the left-back proved to be spot on.
The former Spain coach was eventually sacked after a humiliating 5-1 loss to Barcelona at Camp Nou in October, in which Marcelo was exposed again.
"The board considers there to be a large disparity between the quality within the Real Madrid squad, which boasts eight nominees for the next Ballon d’Or award - an unprecedented number in the club’s history - and the team’s results to date," Madrid said in a pointed statement the following day.
Marcelo was one of the eight players, as was Isco, and three and half months on, it is clear that the blame does not lie solely with Lopetegui.
The Br azilian, voted best full-back in the world by Uefa in each of the last three seasons, is still out of sorts. While most of Madrid’s squad seems to have benefited from the return of fitness coach Antonio Pintus, the 30-year-old still looks to be overweight and mentally, he appears distracted, distant, disengaged.
Out of favour in La Liga and the Champions League, he started in the Copa del Rey semi-final first leg against Barcelona at Camp Nou, but was the weak link once again in that match.
His flank was targeted and he was embarrassed by a surprise starter, Malcom. And when the winger scored, following a move from the left which had been blocked and bounced out, the left-back was still only just arriving in the area when the forward lined up his shot.
Even for a player who has admitted he goes forward and sometimes forgets to track back, that was a bad mistake. Especially given his experience. And against Girona again on Sunday, he was shown up once more as Madrid lost 2-1.
The stats are damning. Of the last nine league matches in which Marcelo has started, Madrid have won only one: a 1-0 win over Rayo Vallecano in December.
In attack, the Br azilian showed some of his trademark skills at the Bernabeu on Sunday and he converted with a shot which was ruled out for offside. First and foremost, though, he is a defender and he endured another tough afternoon in that respect.
After the game, Girona coach Eusebio Sacristan was asked if he had deliberately taken advantage of Marcelo’s moment of form.
He was too polite to say so, of course. "I respect the players a lot and someone like Marcelo is an institution," he said. But right now, any rival coach would logically be targeting the Br azilian.
Later, Solari was asked about the left-back's display and simply said: "Good, I thought he did well."
After the Clasico, the Argentine had said of the Br azilian: "He’s our vice-captain and he is 100 per cent focused on Real Madrid. He thinks of nothing else. We’re working to get the best version of everyone."
That may have been a subtle reminder to the player of his responsibilities. Because on current form, youngster Sergio Reguilon is way ahead of his team-mate.
While Madrid have won one in nine in La Liga with Marcelo starting, Los Blancos have claimed eight victories out of eight with Reguilon in the line-up.
And with Marcelo in the side, Madrid concede on average a goal every 52 minutes. With Reguilon, it is one every 135. It really is not a difficult decision for Solari at the moment.
The 30-year-old called for a run of games in an interview last week, but on his current form, that is too much of a risk. If Real Madrid have serious aspirations of advancing to the final of the Copa del Rey, of challenging for La Liga and of winning the Champions League, Marcelo simply cannot start at the moment.
And that is a shame because, at his breathtaking best, he is one of the world’s most exciting footballers.