Is IP address spoofing possible with ESP transport mode?

Hello IPSec folks,

As far as I know IP header is not protected in ESP mode, so in transport 
mode the source IP may be modified.

Imagine the following scenario:

-Bad guy Bob setup an transport ESP SA with server S. The SA is 
identified by S's IP address, SPI and SA mode (ESP)

-After setting up this SA, Bob send an IP packet protected with this SA's 
parameter, but put Victim Alice's IP address as into source IP address. 
As {dest_IPaddr, SPI, mode} trio point to the valid SA, the packet can 
be decrypted successfully. However the result will be a IP with fake 
source IP address.

Question:
-Can that case really happen or did I miss something?

-There is a source address field in SA. Is it mandatory to check this value 
against the source IP address to protect the above case? I have some 
doubt about it as it can be wildcast in many cases.

-Is there any measure to protect the case described here?

Thanks in the advance,
Son