Valbuena fired in a loose ball from just outside the box in the 25th minute as Marseille recorded a 1-0 victory for the 12th time this season and their seventh clean sheet in a row.
There were angry scenes at the final whistle after Fabien Audard slapped Valbuena in the face.
Lorient goalkeeper Audard and Andre Ayew of Marseille were sent off as tempers flared among both sets of players.
Marseille have 64 points with four games left and are looking good for the second Champions League spot next season.
Coach Elip Baup's Marseille finished Saturday's game with three injured players.
Defender Rod Fanni picked up a thigh problem and is out for 10 days. Striker Andre-Pierre Gignac will miss three or four days of training with an ankle injury and midfielder Benoit Cheyrou has concerns about his thigh.
"This is a very important weekend," Marseille goalkeeper Steve Mandanda told Canal Plus television.
"At this stage of the season the most important thing is the three points and 1-0 is our favourite scoreline."
Valbuena went close to scoring in the seventh minute when Audard palmed away his curling free-kick from 25 metres.
The keeper shone again in the 23rd minute when he kept out a long-range effort from Gignac.
Audard was then beaten by a half-volley from Valbuena after he had parried an effort from Ayew into the path of the French international.
Lorient, seventh on 50 points, pushed for an equaliser in the second half but Mandanda stopped a powerful Maxime Barthelme strike in the 72nd minute and the woodwork denied the attacking midfielder a minute later.
Nancy, who climbed out of the relegation zone for the first time in seven months last weekend, drew 0-0 at Valenciennes and are fifth from bottom on 35 points, one point above safety.
Champions Montpellier stayed eighth on 48 points despite a 2-1 defeat at Ajaccio.
Andy Delort's 82nd-minute penalty for Ajaccio cancelled out Anthony Mounier's second-half opener before Dennis Oliech headed the winner in the last minute.
Ajaccio's Fousseni Diawara was shown a straight red card for a violent tackle in the 47th minute and Montpellier midfielder Younes Belhanda missed the resulting penalty.
Abdelhamid El Kaoutari of Montpellier was also sent off eight minutes from time when he committed the foul that caused Delort's penalty.
