Senegal stretched their unbeaten streak to 24 games, thanks to goals from Ismaila Sarr, Habib Diarra and Cheikh Sabaly.
England's loss on Tuesday (Wednesday AEST), which included a disallowed late goal from Jude Bellingham for a handball in the build-up, was their first defeat in four games under new manager Thomas Tuchel.
"Not good enough," captain Harry Kane told ITV. "We had moments, but with and without the ball things aren't clicking, we're not finding the right tempo. We've lost that aggressive nature that we had."
England, who made 10 changes from Saturday's 1-0 lacklustre World Cup qualifying win over Andorra, took the lead when Kane struck in the seventh minute after Senegal goalkeeper Edouard Mendy saved Anthony Gordon's shot but spilled the ball into the striker's path.
The hosts conceded for the first time under Tuchel when Sarr caught Kyle Walker napping to poke in Nicolas Jackson's cross in the 40th minute.
The visitors doubled their tally in the 62nd minute when Diarra latched on to a ball over the top then fired through goalkeeper Dean Henderson's legs.Â
Sabaly struck deep into stoppage time, prompting boos from England fans after the final whistle.
"We're not going to panic, but we know we need to be better," Kane said. "New ideas, there's new players coming into the team that don't have experience at international level.
"It's a mixture of things, but no excuses. We need to find it quick, the World Cup is going to come around really fast, so every camp is really important right now."
Senegal peppered Henderson with nine shots on target to England's four, including an early header from Sarr that the goalkeeper pushed wide of the post.
Gordon squandered a glorious chance to give England a two-goal lead in the first half when he mis-hit Walker's cross from close range.
Mendy made a great late save to deny England what would have been an equaliser from Bukayo Saka, before Senegal added their third.
"Disappointing result, not sure if maybe we didn't deserve a little bit more result-wise," Tuchel said. "But felt again a bit frozen, not active enough for a long time of the match.''
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