Senegal had been expected to struggle without Mane, who starred as they won the Africa Cup of Nations in January, but they have made it to the last-16 of the World Cup and will face England on Sunday.
“Sadio is obviously an incredible player, and every team would want to have him,” said Southgate.
“But what we’ve seen from the team of Senegal, they’ve become very strong in his absence. And they have shown great spirit through losing him.
“So we know it doesn’t make a difference to the level of the fixture for tomorrow. We’ve got to be prepared for the game, and we’ve got to be at or very best to be able to win the game.
“We’ve been very impressed [with Senegal]. We know that they’re African champions, very proud, great competitive spirit, a lot of belief in the team. With some excellent individual players who cause problems, with a good structure to the team as well. [Head coach] Aliou [Cisse] has done a fantastic job.
“They were very unlucky not to qualify from the group in Russia 2018, and they’ve deservedly done it this time. So we know exactly the size of the task ahead of us.”
The Senegal squad for this World Cup has 10 players playing in England and Southgate believes they have “enriched” the English game.
“We’ve had a number of Senegalese players down the years, from Patrick Vieira to El Hadji Diouf, and the coach,” he said.
“Now, the players who are playing in England now, both in the Premier League, and some guys who are in the Championship but who are really Premier League players.
“They’ve definitely enriched our game. As have players from all around the world, we’re very fortunate that our league is so strong and it encourages the best players in the world to come and play.”
Senegal have been one of the best-supported nations at this World Cup and are expected to get huge backing on Sunday.
FIFA World Cup 2022 Qatar | Best Images and Moments
“We understand there’s going to be a lot of support for Senegal, not only at home but in many of the African nations,” said Southgate.
“Of course we will be considered favourites, Senegal the underdogs, but that doesn’t really mean anything on a one-off game. It doesn’t mean there is no pressure on Senegal because every national team has huge pressure from their own nation, and external as well. I’m sure everybody understands that and knows thats the case”