Moon boots and crutches are often precautionary.
As long as its not a syndesmosis injury and the scans reveal no fracture or ligament damage he will be fine for Arsenal.
If its not high end ligament damage its a 3 week injury if its synd then 6 to 8 weeks.
A player in the AFL limped off badly on Saturday and the early prog was syndesmosis , on crutches and moon boots leaving the ground and scans today mean he will most likely be available for their next game this Friday but certainly the following week.
We err on the side of precauction so I doubt he will play against Chelsea but lets not get too carried away until we are told what the scan reveals if anything other than a bruise.