Depending on your topology, you can have a look at the Health Check Link Aggregation feature (p.271 of 16.1 User Guide). I never really used it, but that might help you.
Another approach could be via ACL. You can create a "sort of" SLB if the servers' IP can be masked correctly. Here's an example of such ACL (you would have several rules like that):
entry r11 {
if {
source-address 0.0.0.0 mask 0.0.0.7;
} then {
redirect-port-list 1:1;
}
}
entry r12 {
if {
source-address 0.0.0.1 mask 0.0.0.7;
} then {
redirect-port-list 1:2;
}
}
