Both players sustained their injuries in the 40-14 loss to Penrith on Saturday night.
Lenihan said that while Campbell's injury was a blow, there was also "some relief" that it wasn't far worse.
"My first thought when I saw it shift was that it was probably an ACL injury," Lenihan told AAP.
"Jayden went down without any contact and the knee shifted a little bit. He will have an extended period off but at least it is not a year.
"The physio felt confident it wasn't an ACL after the game and the scans have shown that he has done some damage to the back of his kneecap."
Lenihan said Sami's injury also was not as bad as first thought and he would not require surgery.
"It might just be a month or two in a boot and he should be fine," Lenihan told AAP.
"Phil Sami should be right for pre-season."
Lenihan said it was likely 23-year-old Campbell would be having an operation.
"These injuries vary a little bit but they are saying three to five months. He is probably due to be back at the start of the season," Lenihan said.
"Jayden has been wonderful for us and has been adaptable in a lot of positions but he will have an extended break now."
The duo join a growing casualty ward. Hooker Sam Verrills was sent for collarbone surgery, which will require a bone graft, but is expected to be fit and firing next year.
Half Tanah Boyd and forward Erin Clark have both had shoulder surgery.
"They won't be able to do contact for four or five months but they will be able to do a lot of running," Lenihan said.
