The second-leg quarterfinal at the San Siro was halted after Inter fans threw flares and bottles onto the pitch following a disallowed goal in the 73rd minute. AC Milan goalkeeper Dida was struck on the shoulder by a flare.
The referee tried to restart play but the match was abandoned when more flares were thrown onto the field.
AC Milan was leading 1-0 in the second leg and 3-0 on aggregate.
Milan has already qualified for the semifinals. But UEFA must still determine whether Tuesday night's result should stand at 1-0 or whether to award the team a 3-0 victory - the standard for an abandoned match.
"In terms of the result there's no suspense but it has to be confirmed officially UEFA spokesman William Gaillard said.
As for sanctions UEFA's disciplinary committee could order Inter to play one or more of its next European home games in an empty San Siro. The team could also face a possible ban from next season's European competitions.
"I think that the Inter people know they deserve a penalty and they will be obliged to play behind closed doors for a few matches AC Milan spokesman Umberto Gandini told Britain's Sky Sports television.
Gandini said the team's Brazilian goalkeeper was not seriously injured by the flare.
"He went after the match to have scans and he just had a bruised shoulder Gandini said.
UEFA also said Wednesday that AC Milan striker Andriy Shevchenko will not be charged for an apparent head-butt on Inter defender Marco Materazzi. He could have faced a ban that would have ruled him out of both legs of the semifinals and the final.
"It appeared that the incident occurred in front of the assistant referee who did not indicate anything to referee (Markus) Merk UEFA said in a statement."