Toney, who has one cap for England as a substitute, is aiming to make his mark in the run-up to Euro 2024, with leading scorer Harry Kane absent due to injury.
Toney has netted goals in 10 games for Brentford since his return to action in January after serving an eight-month suspension for breaching FA betting rules.
Toney was left on the bench during Saturday's 1-0 defeat by Brazil, with Aston Villa forward Ollie Watkins playing the full 90 minutes.
"I think sometimes the quality of (Toney's) football can be underestimated. We're still getting to know all of those strengths because until you work with a player more regularly you don't know them inside out," Southgate told reporters.
"But with England that's the landscape, you don't get hundreds of opportunities. He's a confident guy, he comes in on the back of a good run of form with his club.
"He's going to be on the field on Tuesday, no question. Confidence is crucial. That swagger, that self‑belief. All of the top forward players have it," Southgate added.
"You look at him being a big guy but he's not just a target man. His quality in link play is very good as well. (His penalty record) is definitely a great asset to have. We've obviously got to work out the balance of the squad."
