Hernandez was suspended for Saturday's 1-0 win at Udinese after his red card in the 2-1 defeat at Fiorentina, while Leao was dropped to the bench after that loss which left Fonseca furious.
"They will play tomorrow from the first minute," he told reporters ahead of Tuesday's game at San Siro.
"I expect from them what I expect from everyone, that they give their best to help Milan win. It's a normal situation, even for them. And tomorrow they play."
Samuel Chukwueze scored the winner against Udinese but is set to be among the substitutes for the Brugge game as Fonseca took the unusual step of naming his lineup.
"I will tell you straight away tomorrow's team," Fonseca said with a smile.
"Mike Maignan, Emerson Royal, Matteo Gabbia, Fikayo Tomori, Hernandez, Youssouf Fofana, Ruben Loftus-Cheek, Tijani Reijnders, Leao, Christian Pulisic, Alvaro Morata."
Milan have had a tough start to the Champions League in terms of opposition and results, losing to Liverpool and Bayer Leverkusen, and Fonseca knows this is almost a must-win match.
"We started against two of the strongest teams in Europe right now. They were two different games," he said.
"Difficulties against Liverpool, quality against Leverkusen, especially in the second half. We didn't win but we showed signs of growth.
"We have to continue to grow in this competition and we want to win if we want to have a chance to continue. It's not decisive but it is important, we have to win."
Milan have a shortage of strikers. Tammy Abraham went off injured minutes after coming onto the pitch on Saturday, while Luka Jovic was not included in the Champions League squad.
Francesco Camarda has been called up from the youth squad and if he were to come on and score at 16 years and 226 days he would become the youngest-ever scorer in the Champions League.
"This is not a normal situation to have Abraham and Jovic out," Fonseca said.
"Normally we always have solutions, plus we have Camarda in who we believe a lot. I don't think we need other players in this position."