England have no injury issues going into the game with forward Marcus Rashford having overcome a minor knee problem which prevented him training earlier in the week.
“They’re ready,” Southgate said.
“If they played today they’d be ready.
“We’ve got to make sure they don’t do too much physically, don’t overload them with information. They’re clear on how we want to play, they’re clear on the opposition.”
England seemed in relaxed mood on the training field, with the players stopping for a moment to applaud midfielder Jordan Henderson on his 28th birthday.
Southgate has already told the group what Monday’s starting line-up will be.
“I think that’s helpful that everything’s settled,” he said.
“They know their jobs. The guys who aren’t in the team are prepared and have trained well. They become a key part of what we do now.
“They have to support the players who are starting but also be ready because anything can happen before the game, anything can happen during the game.
"We only progress as a collective. That’s been the mentality from the day we started and will go right through the tournament.”
Meanwhile, Tunisia manager Nabil Maaloul believes England’s team spirit under Gareth Southgate makes them a superior proposition to the star-studded sides of years gone by.
The Group G rivals kick-off their World Cup campaigns in Volgograd on Monday night, reprising a fixture which was last contested at the same stage during France ‘98.
“It is true that Tunisia played against England in 1998, when they had great players like Beckham and Owen, but there was not the same great collective spirit,” he said.
“I think that this team has progressed enormously.
“They have young players, the third youngest they’ve sent to a World Cup. Maybe they’ve changed a bit in terms of their mentality, their attitude.”