It was fourth-placed Marseille's fourth league defeat in a row and, with only four matches left, qualification for next season's Champions League as a top-three finisher is also now in danger.
"We were absent in the first half and disorganised in the second," said Argentine coach Bielsa.
First-half goals from Jordan Ayew and Francois Bellugou set the tone although the hosts dragged themselves back to 2-2 with strikes by Andre Ayew and Jeremy Morel.
Within a minute of Morel's equaliser, Romain Philippoteaux put the visitors in front again.
Michy Batshuayi made it 3-3 in the 76th minute but Jordan Ayew bagged his second goal with six minutes to go and Mathias Autret delivered the coup de grace that lifted Lorient out of the relegation zone.
Marseille were top of the table until Christmas and since then their form has dipped alarmingly, leaving them eight points behind leaders Olympique Lyonnais having played one game more.
Bielsa's men lost to mid-table Nantes last week and were without suspended playmaker Dimitri Payet.
Payet was back on Friday but could not help stop his team's run of defeats.
"Things haven't been easy recently," said coach Sylvain Ripoll after Lorient climbed one point clear of the drop zone.
"It's not done yet and we need to build on what we've done on a night like tonight."
On Sunday, Lyon (65 points) travel to Stade Reims and third-placed Monaco (59) visit bottom of the table Racing Lens.
Second-placed Paris St Germain (65 points), who have a game in hand on Lyon, host Lille on Saturday.
(Reporting by Andrew Callus, editing by Tony Jimenez)
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