City have confirmed that the highly-rated youngster, who spent last season at Lille, has moved to the Ligue 1 side in a deal thought to be worth around £9million.
The news came just minutes after Pellegrini, speaking in his weekly press conference, had said the move would be a temporary one.
Pellegrini said: “Marcos is still a player here for Manchester City. We will see his future in the next hours.
“I think Marcos needs to play more in another club. He is a very good player. I am sure in the future he will be part of our club, but for this year he must continue trying to play as much football as he can.
“Here he has a lot of competition with David (Silva) and Samir (Nasri) so it is better for him to go on loan.”
That information was soon obsolete as City announced on their website the 20-year-old Portuguese midfielder was moving on.
“Marcos Lopes has joined AS Monaco on a permanent deal,” the news story read.
Lopes was regarded as one of City’s most promising youngsters but made just five appearances after joining the club from Benfica in 2011.
He made an immediate impact with a goal on debut – becoming City’s youngest-ever goalscorer at 17 years and nine days – in an FA Cup tie against Watford in January 2013.
The speed his move was concluded may even have caught out some of his team-mates, with Lopes tweeting: “Sorry I couldn’t say proper bye to my mates @MCFC but I’m sure our ways will meet again sometime soon. I know you will win many titles!”
In further news, Spanish second division side SD Ponferradina have announced that City have signed striker Ruben Sobrino for a fee of £200,000 with a further potential £150,000 in add-ons. It is thought the 23-year-old is likely to head straight out on loan.
As for the major transfer news surrounding City, Pellegrini was unwilling to speak about the club’s interest in Wolfsburg’s Kevin de Bruyne.
Klaus Allofs, director of sport at the German club, has revealed City have made an “astonishing” offer for the 24-year-old former Chelsea playmaker and discussions are thought to be ongoing.
But Pellegrini, who has consistently refused to speak about transfer targets, said: “You must continue asking but the answer for the question is the same. I don’t talk about rumours.”
Pellegrini was similarly unwilling to speak about his side’s testing Champions League draw, which has pitted the club against Juventus, Sevilla and Borussia Monchengladbach. The Chilean deflected a question on the matter by referring to Saturday’s upcoming Barclays Premier League clash with Watford.
“All the groups are difficult and the most difficult team in the group is Watford,” he said.
Pellegrini confirmed striker Wilfried Bony would miss the game after suffering a gash in his leg of around six or seven centimetres.
Bony had earlier posted a photograph of himself wearing a protective boot but the problem is not too serious.
Pellegrini said: “He is not able to play tomorrow but, with the international break, he will be ready for the next game.”
Left-back Gael Clichy has further problems with his ankle and is likely to be out another six to seven weeks.
City are looking to maintain their 100 per cent start to the campaign and can set a new record of 10 successive wins by beating the promoted Hornets.
Pellegrini said: “It is a difficult game. They have played three games in the Premier League and three games they have drawn.
“They have a very good manager and good players and the great mistake we could make in this moment is to think that, because we won the last three games, we will win this game.”