Just found out on atp website, it was a neck muscle strain. Hopefully only precautionary and Wawrinka's fine to go for the AO.
Sounds like one of those "non injury-injuries" that players use as an excuse to pull out of an event when they reached the final of an event the previous week and basically just need the time to rest and practice.
Similar to Henin pulling out of Sydney after a tough 3 set final in Brisbane and used muscle tightness in her left leg as an excuse to get some rest and practice in before AO.
Both players will be perfectly fine.