The pair were both substituted during Hull's 1-0 defeat to West Brom on Saturday, and scans have shown the extent of their problems.
Jelavic was substituted after 33 minutes at The Hawthorns having taken a knock to his knee, and scans have shown he will be out for between three and six weeks despite suffering no serious damage.
Hernandez limped off six minutes later, with the club confirming on Tuesday he had suffered a small tear of his abductor muscle, ruling him out for up to six weeks.
The duo are the latest to join a growing injury list at the KC Stadium with Mohamed Diame (knee), Andrew Robertson (ankle), Liam Rosenior (hamstring) and Robert Snodgrass (knee) all long-term absentees.
However, Steve Bruce could have Gaston Ramirez and Sone Aluko back for their trip to West Ham on Sunday.
The pair picked up groin and calf injuries respectively in recent weeks and had been expected to be sidelined for a significant period of time, however both are making quick recoveries.
A club update confirmed Snodgrass - injured 40 minutes into his league debut for the club - will not return from a dislocated kneecap before the beginning of next season.
