Asus Sabertooth 990FX supports only AM3 and later chipset processors. It doesnot support a AM2+ processor.
The "920" is an odd-duck, but the CPU Support list includes every X4 from the 805 to the 980 (including the 910 and 925 by name). I rarely find the "920" mentioned specifically.
But that's immaterial. Every Phenom II X4 from the "965" on down is an AM2/AM2+ processor, and the Sabertooth claims to support it.