F5 with out default gateway

The BIG-IP has a feature called auto lasthop, which is on by default. That feature causes the BIG-IP to return response traffic to the same MAC address that the traffic originated from,in your case the ASA.

Where you would run into problems is if the servers initiated outbound traffic, that would fail due to the lack of a route, and the lack of a forwarding IP virtual server if one does not exist.