Marshall, a sophomore from Bossier City, Louisiana, leads the No. 4 Tigers with six touchdown catches, which is tied for second among all FBS players. He has 20 catches for 304 yards.
Tigers coach Ed Orgeron said after Saturday's victory over the Commodores that Marshall's injury "doesn't look good, for now."
Injuries are beginning to pile up for the Tigers, who are about to hit the meat of their schedule, after this coming weekend's bye week.
Wideout Justin Jefferson (ankle) and outside linebacker Michael Divinity Jr. (lower left leg) also were hurt in the first half at Vanderbilt. Orgeron said Divinity's injury was "pretty significant" but "no breaks or nothing." Divinity's status wasn't known on Sunday. Jefferson posted on social media that he had suffered only a minor ankle sprain.
The Tigers already were missing several key players, including outside linebacker K'Lavon Chaisson (ankle), who didn't play at Vanderbilt, and defensive linemen Rashard Lawrence (ankle) and Glen Logan (foot/ankle), who missed the previous two games. Safety Todd Harris Jr. (knee) is out for the season and will redshirt.
Along with Ja'Marr Chase, Marshall and Jefferson are among a trio of wide receivers who have helped the Tigers lead the FBS in scoring with 57.8 points per game. LSU ranks No. 2 in the FBS in passing (431.8 yards) and No. 5 in total offense (563.5 yards).
The Tigers will host Utah State on Oct. 5, before opening a difficult four-game stretch against SEC opponents: vs. No. 9 Florida on Oct. 12; at Mississippi State on Oct. 19; vs. No. 7 Auburn on Oct. 26; and at No. 2 Alabama on Nov. 9.