Jadon Sancho and Raphael Guerreiro will be assessed before a decision is made on whether they can play any part in Borussia Dortmund's Bundesliga showdown with rivals Bayern Munich.
England international Sancho hobbled off the pitch two minutes after scoring what proved to be the winning goal in Tuesday's 1-0 DFB-Pokal quarter-final win against Borussia Monchengladbach.
He watched the final 22 minutes from the bench with an ice pack on his upper leg, with interim head coach Edin Terzic confirming after the match the 20-year-old had sustained a muscular issue.
Guerreiro, used 29 times in all competitions this season, limped off in the opening five minutes of the cup tie and both players are now doubtful for Saturday's Klassiker at the Allianz Arena.
"Jadon and Raphael were examined immediately after the Gladbach game and we have to carry out further examinations," Terzic said at a pre-match news conference on Thursday.
"It could be tight for both of them, but we're not giving up hope that they will fly to Munich with us tomorrow."
Being without Sancho would be a major blow for Dortmund, who are looking to avoid losing a fifth league game in a row against Bayern for the first time since between 1968 and 1970.
After a largely underwhelming first half to the campaign, the attacking midfielder has been directly involved in 12 Bundesliga goals in 2021, finding the net six times and setting up six more.
Only Bayern striker Robert Lewandowski has scored and assisted more this calendar year (13 in total – 11 goals, two assists), with Eintracht Frankfurt duo Filip Kostic and Andre Silva next on the list with 10 goal involvements apiece.
Lewandowski has 28 Bundesliga goals to his name in total this campaign - just 12 short of Gerd Muller's long-standing record in a single season - for a Bayern side on a 36-game run since last failing to score in the league.
No team in Europe's top five leagues has found the net more times than Bayern (67) in 2020-21 and Terzic, who will be replaced by Marco Rose at the end of the campaign, acknowledged his side will have to be alert at the back this weekend.
"It is clear that the toolbox of the opposition is very large," he said. "We will try to support each other and block the flanks and cover the posts.
"They score a lot of goals. It would be best if we don't let them get into the final third of the field.
"There will be phases where we have to suffer. That counts when you play in Munich, but it does not rule out being brave yourself and being dangerous. Small things will make the difference in such close matches."