Barcelona would be allowed to reinstate Marc-André ter Stegen to their Champions League squad this season but only at the expense of Wojciech Szczęsny, sources have confirmed to ESPN.
Ter Stegen injured his knee last September and his place in the Barça squad was taken by Szczęsny, who came out of retirement to join the club in October.
Szczęsny has gone on to become Barça's No.1, seeing off competition from Iñaki Peña, and the Catalans are unbeaten in the 19 games he has played.
However, Ter Stegen returned to light training this week ahead of schedule, raising the possibility of a return to action before the end of the campaign.
Both goalkeepers are eligible to play in LaLiga, but the scenario is slightly more complicated in the Champions League, where Szczęsny was registered in Ter Stegen's position.
UEFA's rules state: "Once the injured or ill goalkeeper is fit to be fielded again, he can resume his position in place of his substitute.
"The return of the original goalkeeper must be announced to the UEFA administration 24 hours before the next match in which the original goalkeeper is due to play."
There was some confusion as to whether that was still applicable given another registration window, in January, had passed for UEFA competitions, but a source confirmed it is still the case.
Therefore, should Barça wish to use Ter Stegen in the Champions League again this season, they would have to inform UEFA of the change, which would subsequently make Szczęsny ineligible for European competitions.
"We will see, step by step," Flick said of Ter Stegen's chances of playing again this term on Tuesday. "We have to wait and see what happens. It's no time to make a decision or say anything about that.
"The good news is that he's in a good moment. Everything is going in the right direction and we are very happy he's in this stage. We will see what happens."
Barça meet Borussia Dortmund in the quarterfinals of the Champions League, with the first leg taking place in Barcelona next week.
In total, Barça could have another 16 games to play this season: nine in LaLiga, five in the Champions League and two in the Copa del Rey.