The two will go head to head as Milan take on Newcastle United in the group stages of the competition.
“Obviously it will be nice to see him again. We played a lot of games together," Tomori said in his pre-match press conference of Tonali, who joined Newcastle from Milan this summer.
"However, there will be no pleasantries once the teams step out on to the pitch.
“It will be nice to see him again so soon after he left, but we are professionals. When the game starts, friendships go out of the window. We want to win. After the game we can start being friends again. During the game, though, we are not friends."