Flekken, the Netherlands international and Leverkusen's first-choice goalkeeper, suffered a knee ligament injury during the match. Tella, a Nigeria international, picked up a foot injury after coming on as a substitute in the second half.
The club issued a brief update on Sunday, making it clear that neither player will be returning quickly. "Both players will not be available for a longer period of time," Leverkusen said in an official statement.
The injuries come at a challenging moment for head coach Kasper Hjulmand. With a busy schedule ahead, Hjulmand now has to adjust his plans at both ends of the pitch.
Flekken's absence leaves Leverkusen without their established presence in goal, while Tella's injury reduces attacking options, particularly in wide areas where his pace and energy have been valuable.
Leverkusen are currently sixth in the Bundesliga standings and travel to face Olympiacos in the Champions League on Tuesday. Squad depth will be tested as they look to respond to the Hoffenheim defeat and maintain momentum in Europe.
With no clear return dates given, Leverkusen supporters will be hoping both players recover quickly as the season enters a demanding phase.
Harry Kane made it clear that he has no concerns about Bayern Munich losing momentum if they wrapped up the Bundesliga title early, insisting the squad's mentality would not change under coach Vincent Kompany.
Hugo Ekitike has admitted he was relishing the chance to play at the Orange Velodrome again as Liverpool prepared to face Marseille in the UEFA Champions League on Wednesday night.
Bundesliga club Eintracht Frankfurt parted ways with manager Dino Toppmoller after they failed to beat Werder Bremen over the weekend.
Bayern Munich recovered from a goal down to thump RB Leipzig 5-1 thanks to a blitz of goals in the second half at Red Bull Arena.
