The African side had looked to be the better team in a goalless first half before France captain Kylian Mbappe scored in the 66th minute and Bradley Barcola added another goal in the 82nd. The joy of Ibrahim Mbaye's stoppage-time strike was cut short as Mbappe sent in another from distance a minute later.
Senegal return to New York/New Jersey Stadium on Monday to play Norway.
"There were some mistakes, it was a collective mistake," said Thiaw. "Sometimes we had a low line block and we were very passive on the pitch and we need to be more aggressive for the ball carriers so that certain passes don't happen like (when France scored)."
Senegal entered the World Cup on a solid run, having won the Africa Cup of Nations before the Confederation of African Football (CAF) controversially stripped them of the title earlier this year.
"We need to remobilise the players - it's been a long time since this team has lost a first match in a competition but we have two more matches and we still have our chances and we need to focus very well for the next match so we can get the three points," said Thiaw.
Senegal face another prolific scorer in Erling Haaland when they play Norway.
"We need to keep working on that to keep the lines closer," said Thiaw. "This will enable us to have a more compact block, not to work left to right."
FIFA World Cup 2026
The 2026 World Cup is taking place from June 11th to July 19th in the United States, Canada and Mexico. The tournament features 48 national teams and is played at 16 modern stadiums.
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