The Brazilian has lost his place in the starting XI in recent times, with reports in the Spanish and Italian press claiming that he is keen to leave in the summer and link up with Cristiano Ronaldo at Juve.
Marcelo, a mainstay at the Santiago Bernabeu since joining from Fluminense in 2007, has not started a game in his favoured left-back slot since the 2-0 defeat at home to Real Sociedad on January 6, with Sergio Reguilon seemingly now first choice.
Indeed, Reguilon has only been promoted from the club's Castilla side this season but has made 10 appearances in all competitions.
He told a news conference: "His love for the club and his commitment to the club and his joy at training are undoubted, and his behaviour is impeccable. All these things must be praised.
"It's always a difficult task to pick the team."
Asked if he had discussed the situation with Marcelo privately, he added: "What happens in private stays in private.
"But competition within the squad is an essential part of football. Everyone must feel they have an opportunity and can lose their place [if they don't do well]."
Madrid failed to win a domestic competition last season - apart from the season-opening Super Cup - and Solari is eager to improve that record, although he admits 12th-placed Girona will be no pushovers.
He said: "Girona are having a very good season, they're very dangerous on the counter-attack.
"We want to repeat what we showed against Sevilla [on Saturday], when we played a very complete game, and we must bring that to the team in the Copa."