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."
Real Madrid have received a significant lift ahead of their Champions League round-of-16 second-leg clash with Manchester City on Tuesday, with both Kylian Mbappe and Jude Bellingham included in the squad traveling to England.
Chelsea's pursuit of a place in next season's Champions League has entered a pivotal phase, and now head coach Liam Rosenior has found himself under increased pressure.
Vincent Kompany admits replacing Michael Olise will not be easy after the winger's display helped Bayern Munich secure a commanding advantage in their Champions League tie against Atalanta.
Luis Diaz was both hero and villain for Bayern Munich in the Bundesliga on Saturday as he scored the equaliser before being sent off as the Bavarians drew 1-1 with Bayer Leverkusen at the BayArena.
