Celtic centre-half Virgil Van Dijk says he must learn from the red card he was shown in the but feels the referee was badly mistaken in the San Siro.
Slovakia's Ivan Kruzliak booked the Dutchman for a foul on Rodrigo Palacio and, within 10 minutes, sent him off for a clumsy challenge on Mauro Icardi.
"I got a red card and I feel horrible. I need to learn from it. It's going to be tough," Van Dijk told BBC Scotland.
"The referee was horrible today."
The first booking meant Van Dijk would have missed Celtic's Europa League last 16 first-leg tie had they gone through.
The second yellow was awarded on 36 minutes when Icardi looked to have got the better of the imposing defender and was trying to race toward Craig Gordon's goal.
It also made Celtic's challenge so much harder, with the game balanced at 3-3 from the first leg at Celtic Park.
Inter's Fredy Guarin scored the only goal of the second leg in the 88th minute to seal a 4-3 aggregate victory.
"I want to apologise to my team-mates and to the fans who travelled," Van Dijk continued.
"It was never a red card and everybody could see that, but he gave it. It's a terrible feeling for me now.
"If you look at the fouls, the first one was never a yellow card. It was maybe a foul - but never a yellow card.
"The second one was a 50-50 challenge in the air and we both go with the shoulder and he gave a yellow card because I think the striker asked for it. It is very disappointing.
"It is very difficult with 10 men to score a goal. After the red card, you're going to get trouble. They scored a beauty and the game was over.
"Now you're the worst player Celtic have, that's what they're going to say right now, but another day, you'll be the best player. That's an awful feeling."
Van Dijk also believed Celtic midfielder Stuart Armstrong should have been awarded a penalty for being pushed in the back inside the Inter box.
The Scottish Premiership leaders have a home league match against Aberdeen on Sunday and, with a League Cup final against Dundee United to look forward to and a Scottish Cup quarter-final against the same team, Van Dijk's attention is now on the domestic treble.
"We have a big chance to do it and we want to do it," he said.
"It's a big game on Sunday and we need to focus on that. Come Saturday, we'll be focused and we want to win on Sunday with a good performance.
"Aberdeen have a couple of good players, but I think we have a better team and, if we are 100%, we have the biggest chance to win the game."
For team-mate Stefan Johansen, the performances against the Italians were a sign of progress.
"We were not at the Europa level in the beginning of the season, but I think over two games against Inter we showed that we are there now and can compete with the very good teams," he said.
"Now we want to focus on Scotland for the rest of the season. We now know how demanding it is to play games like this. We need to work even harder.
"The team has adapted. We are better now, so things are looking good for Celtic."
Celtic striker John Guidetti was frustrated by the referee's decision, arguing that "the game should be settled by the players, not by someone else".
He added: "I can lose a football match. I don't like it, but I can lose a football match if we play a fair game and the opposition is better than us and wins fair, but I don't think that was the case."