The major news before kick-off was that Barcelona failed to register star summer signing Dani Olmo, leaving the Spain international to watch his new teammates from the stands at Lluis Companys Olympic Stadium.
There was also the small matter of Nico Williams facing a club he has been heavily linked with throughout the summer transfer window.
Nico Williams could do nothing to prevent his Spain teammate and friend Lamine Yamal from breaking the deadlock with a deflected strike.
However, Barcelona were unable to protect their slender advantage, with Pau Cubarsi's foul giving Oihan Sancet the opportunity to slot home from the spot.
The Blaugrana appeared to be on course for a frustrating evening until Lewandowski restored his team's advantage after previously going close on several occasions.
Lewandowski pounced onto a loose ball from Pedri's parried cross before he fired a left-foot shot past Alex Padilla to clinch all three points.
The win takes Barcelona onto six points from two matches, while Athletic are left with just one point from their opening two games.
BARCELONA VS. ATHLETIC BILBAO HIGHLIGHTS
Lamine Yamal goal vs. Athletic Bilbao (24th min, Barcelona 1-0 Athletic Bilbao)
The focus may have been on Nico Williams prior to the contest, but it is his Spain teammate who breaks the deadlock at Monjuic. Yamal cuts inside onto his favoured right foot before beating Padilla with the help of a deflection.
Sancet goal vs. Barcelona (42nd min, Barcelona 1-1 Athletic Bilbao)
The referee Jesus Gil Manzano is sent to the monitor to review Cubarsi's desperate challenge on Alex Berenguer. The official swiftly reviews the footage and points to the spot, allowing Sancet to step up and slot home his second goal of the season.
Lewandowski goal vs. Athletic Bilbao (75th min, Barcelona 2-1 Athletic Bilbao)
Lewandowski finally gets his goal after going close on a number of occasions. Pedri's driven cross is parried into the striker's path, and he makes no mistake in firing the ball into the awaiting net.
MAN OF THE MATCH - ROBERT LEWANDOWSKI
For some time, Lewandowski looked destined to experience a frustrating evening after being denied by the woodwork on a couple of occasions while also seeing a goalbound strike saved by Padilla.
However, the Poland international was ultimately rewarded for his persistence, latching onto a loose ball to fire him his third goal of the season.
Just like the win against Valencia, Lewandowski proved to be the decisive figure on the pitch and is ultimately deserving of the man of the match award.
BARCELONA VS. ATHLETIC BILBAO MATCH STATS
Possession: Barcelona 66%-34% Athletic Bilbao
Shots: Barcelona 13-9 Athletic Bilbao
Shots on target: Barcelona 5-2 Athletic Bilbao
Corners: Barcelona 5-6 Athletic Bilbao
Fouls: Barcelona 2-5 Athletic Bilbao
BEST STATS
WHAT NEXT?
After beginning the season with consecutive 2-1 victories, Barcelona will quickly turn their focus to Tuesday's away fixture against Rayo Vallecano.
As for Athletic, they will attempt to claim their first win of the season when they welcome Valencia to San Mames on Wednesday.