The young forward scored the decisive goal in the second half, shifting the match in Bayern's favour after the German side had fallen behind shortly after the interval.
His influential performance earned him UEFA's Man of the Match award, reinforcing his status as one of the club's most exciting emerging talents.
Bayern's response to going a goal down was immediate and assured, with Karl's strike sparking a confident surge that ultimately delivered all three points. His composure in key moments was a defining factor in Bayern's turnaround.
Head coach Vincent Kompany was full of praise for Karl after the match, commending not only his decisive contribution but also the consistency he has shown throughout the season.
Kompany remarked that Karl has delivered even better performances in previous outings, highlighting his impressive maturity and growing influence within the squad.
"Lennart is always dangerous. To be honest, he's played better games for us, but he's always there," Kompany told DAZN.
"When he gets his moments, he just finishes. That's important for us."
He added: "Sporting is a tough opponent; they're in the top eight of the Champions League for a reason. We stayed calm after going behind."
Bayern Munich have moved closer to welcoming Jamal Musiala back into action as the Germany international continued his steady return from the serious leg injury he suffered during the Club World Cup in July.
Athletic Bilbao held Champions League holders Paris Saint-Germain to a goalless draw on Wednesday, a result that preserves the Basque side's hopes of securing a playoff berth.
Wednesday night's Champions League action saw Borussia Dortmund slip up against Bodo/Glimt, while Bayer Leverkusen earned a late draw against Newcastle United.
Chelsea head coach Enzo Maresca has urged his players to move on swiftly from their UEFA Champions League defeat to Atalanta, insisting the focus must now shift to Everton.
