Milan defender Tomori, 24, was dismissed after getting the wrong side of Chelsea playmaker Mount, 23, and pulling him down in the box. Referee Daniel Siebert pointed to the spot and showed the former Blues defender a straight red card for the offence.
Some critics have deemed Tomori's red card as harsh due to the minimal contact between the two players. Even Chelsea manager Graham Potter has admitted his team were "lucky", as it helped them record a second successive Champions League win.
"He's obviously my mate, I've known Fik for a very long time and I felt for him in that situation, I was gutted for him," Mount told Chelsea's website. "I don't think it was a sending-off. Was it a penalty? Maybe yes.
Mount added: "I felt him trying to pull me back, but I was running to goal so I wasn't going to go down and I felt like I could still have an opportunity to score. So I carried on going, didn't score and then obviously the ref blew up and gave a pen straight away.
"I was probably a bit surprised at the time because I've still gone through and had a shot. It was hard to see him get sent off.
"You still want to be playing 11 versus 11 and going at it with two good teams, so it maybe ruined the game a bit. It was massive for us and we obviously wanted to win the game, which we did, but I still feel for Fik a lot."
Jorginho scored the penalty before Pierre-Emerick Aubameyang's strike killed the contest. The Blues are now top of their group after their poor start to the tournament - picking up just a point from their first two games against Dinamo Zagreb and Red Bull Salzburg.
Speaking to BT Sport after the game, Potter said: "To come here and win is not easy. Credit to the players. They were really good.
"We had a bit of luck with the opposition going down to 10 men. Overall the performance was good. It's nice to keep a clean sheet and to come away with three points is fantastic."
On the red card and penalty decision, Potter added: "It's hard from where I'm sitting. It looks like he had his hand on him and Mason Mount was through on goal. I need to look again clearly. It's nice for the penalty and nice for Jorginho to score.
"It was a nice move to lead to it. It is what it is. Sometimes they go for you. It's tricky. You want to control the game but not to be too passive. With the atmosphere here, one action can result in a goal, the stadium erupts and they feel like they have an extra man.
"I'm an honest player and I want to try to score a goal and he pulled me back a little bit, but I feel for him for that decision. It's a tough one."
Mount and Tomori came through the ranks together at Stamford Bridge after they both joined the club's academy in 2005. They both went on loan to Derby County during the 2018/19 campaign - helping Frank Lampard's side reach the Championship play-off final.
Mount and Tomori both broke into Chelsea's first team the following season before the latter joined Milan, initially on loan, in January 2021. Tomori subsequently missed Chelsea's 2021 Champions League triumph but won the Serie A title at San Siro last season.