Marrone elected against going into specifics about the injury that Lee sustained during Saturday's 17-6 preseason win over the Atlanta Falcons.
A source told NFL Network's Ian Rapoport that Lee suffered ligament damage.
Lee was injured after being tackled by Falcons safety Damontae Kazee following a 20-yard catch during the first quarter. Lee's left knee buckled and he immediately clutched at it with both hands.
Kazee was penalized on the play for lowering his helmet to initiate contact.
The entire Jaguars wide receiver corps was present on the field as Lee was carted off.
Signed to a four-year, $34 million contract ($16.5 million guaranteed) in March, Lee has recorded 171 receptions for 2,166 yards and eight touchdowns in four seasons with the team. He led the Jaguars in catches (56) and was second in yards (702) in 2017.
With Allen Robinson (Chicago Bears) and Allen Hurns (Dallas Cowboys) having departed in free agency, the Jaguars feature a mostly inexperienced group of wide receivers. That core includes Donte Moncrief, Keelan Cole, Dede Westbrook and second-round pick D.J. Chark.
"Everybody's got to step up," said Moncrief, who joined the team in March on a one-year, $9.6 million guaranteed contract.
"Everybody in the room's got to be ready to go, from the young guys to me. He was one of the big roles in the offense."