Already missing Kerr and US striker Mia Fishel the London club lost England's Lauren James on the eve of the match, leaving them light in attack against the holders in their Uefa Women's Champions League semi-final first-leg.
While Barcelona were able to bring the competition's leading scorer, Claudia Pina, off the bench Chelsea had fewer attacking options.
Pina scored twice as Barca struck three times in the closing 20 minutes to secure an imposing first-leg lead.
With Ellie Carpenter's Lyon winning 2-1 at Arsenal on Saturday a repeat of last year's final, won by Barcelona, looks likely.
The Spanish champions could even afford a missed penalty by former world player of the year Alexia Putellas, who was denied by Hannah Hampton after nine minutes.
Polish striker Ewa Pajor broke the deadlock in the 35th minute for her 36th goal in 39 games this season. In the second period Pina doubled the lead, Sandy Baltimore halved it, then Irene Paredes and Pina made it commanding.
Chelsea's former Barca player Lucy Bronze insisted her current team could overturn the odds at home next Sunday. "I think we can do it," the England defender Bronze told TNT Sports. "Never say never. Nothing is impossible."
That echoed Australian striker Caitlin Foord's words on whether Arsenal can win in Lyon next weekend.
However, a third Barca-Lyon final in four years seems probable in Lisbon on 24 May.
with agencies
