Alaba sat out Bayern’s final training session on Tuesday with a thigh strain, but coach Jupp Heynckes says a decision will be made early on Wednesday as to whether he will be in the squad.
Midfielder Corentin Tolisso is also a doubt while Brazilian defender Rafinha is likely to replace Alaba if he fails to make the squad.
On-loan Real Madrid midfielder James Rodriguez could start after being used as a substitute in the 3-0 victory over Hannover on Saturday.
Madrid have no fresh injury concerns.
Bayern Munich coach Heynckes helped re-establish Real Madrid’s reputation as kings of Europe by winning the Champions League in 1998 after a 32-year drought.
But his memories were not fond ones as he was sacked eight days later when his side finished fourth in the league during his single season in charge.
Now Heynckes’ former club stand in the way of his bid to become only the third coach alongside Carlo Ancelotti and Bob Paisley to win Europe’s biggest prize three times.
Bayern, who have already won the Budesliga title and the German Cup, are hoping to complete the treble in Heynckes’ fourth spell in charge.
But this time Bayern are the underdogs as 12-time champions Real Madrid are looking to win their third successive Champions League trophy under Zinedine Zidane.
“You have to look back to last October,” said Heynckes. “We did not think we were going to be champions with five games to spare and to be semi-finalists (in Europe).”
“These are two teams with a great tradition in European football, who play and love attractive football.
“It’s a difficult draw for both sides,” he added.
The Bayern coach also played down the threat of Real Madrid top scorer Cristiano Ronaldo, who has scored in all 10 of their European matches this season – taking his tally to 42 goals in all competitions.
He scored five goals over two legs in Madrid’s 6-3 aggregate win over Bayern in the 2017 quarter-finals.
“Of course, he’s a big topic for us in the game analysis,” said Heynckes.
“But don’t forget we have Robert Lewandowski, who has scored 39 goals. So you have to ask, how can you stop Lewandowski?
“There are world-class players on both sides.”
Madrid coach Zidane said his side were going “to think positively” despite coming under criticism in recent weeks for missing a number of clear goalscoring chances.
“I’m not worried,” said Zidane. “It’s true that in the last two games we have had chances and we have not taken them but now we are going to have a game on Wednesday away from home.
“We are going to try to score and realise that this is football anything can happen, no matter who is playing on the pitch.”
German football writer Raphael Honigstein on the BBC Radio 5 live’s Euro Leagues Football Show
“Real Madrid are favourites, I think it’s a 60-40 situation. But Bayern will look at the other semi-finalists and think ‘they’re all there because they upset the odds along the way. Why can’t we do it?’
“I think James Rodriguez will play – he didn’t start at the weekend at Hannover which I think was a clear indication he will start.
“Bayern have shown very little of the fluidity and out-of-this-world football we’ve become accustomed to in this spell but they’ve almost become the Bayern of old – they somehow find a way of winning games.
“The mindset has changed and become more efficient and in the moment. They don’t dominate games like the unbelievable football of Guardiola, but they are just as good if not better at making the most out of the key moments.
“They will take some belief from what Juventus have done. Heynckes will never go all out. He’ll say ‘let’s keep it really tight and not concede if possible.”