Pep Guardiola revealed that he is "still angry" with Kyle Walker over the sending off which means he will miss Wednesday's last-16 second leg against Sporting.
Walker was shown a red card in Manchester City’s final group stage game away to RB Leipzig in December, a dead rubber which ended in a 2-1 defeat.
The 31-year-old was handed a three-match ban by Uefa for needlessly kicking Leipzig's Andre Silva, which Uefa deemed to be an 'assault'.
City saw an appeal against Walker's suspension rejected by Uefa and Guardiola said he did not agree with the attempt to reduce the three-game ban.
"He deserved the three-game ban. When a player does a stupid thing, he deserves to be banned for three games. I'm not so kind to Kyle in this kind of action. I'm still so angry with him,” the City manager said.
"It's not necessary to discuss with him, he knows it. Kyle is so important for us, you saw the game he played against United? But in Leipzig when we had already qualified, 80 minutes in and he made this kind of action?
"He deserves the three games. The club appealed but I did not agree. They have to do it because in other similar cases it was just two games [but] he deserved it. Hopefully he learns for the future."
Walker will serve the second part of his three-game ban on Wednesday night, with City 5-0 ahead on aggregate, and will also miss the first leg of the quarter-finals.
The right-back’s absence means that Guardiola is short on options in defence, with Joao Cancelo laid low by an illness. Ruben Dias and Nathan Ake are also absent through injuty.
“We don’t have many players. We have just 14 players available. We have a few problems, the suspension for Kyle, Joao is sick and cannot play. We are in a little bit of trouble behind, in the back four,” Guardiola admitted.
