You might also try an HTTP 1.0 send string without a host header:
HEAD /index.htm HTTP/1.0\r\nConnection: Close\r\n\r\n
For 10.2+ you should use two \r\n's at the end of the send string. For older versions, just one:
sol10655: Change in Behavior: CR/LF characters appended to the HTTP monitor Send string
http://support.f5.com/kb/en-us/solutions/public/10000/600/sol10655.html
If that doesn't work, you can set a host header with an inbuilt HTTP(S) monitor. However if it needs to be specific to the server being polled, you'd need to create a custom monitor for each pool member. As Nitass said using an FQDN should be a simpler solution.
Aaron