Plaster question - prior to tiling

[quatro]

We have removed the tiles from kitchen walls - in many places the finishing layer of plaster came off with them and the walls need skimming.
The plaster that is remaining is quite soft though - it's stuck to the walls ok but can be easily dug into with a screwdriver.
Will it be ok to skim over this?....or does it need removing because it is too soft?
Not come across this prob before.....

 

[Third_Eye]

Try and send a pic if u can ?