I think the reason for that was because he was never actually "given a number" by Lambert, instead just assigned a spare one by someone at the club as, due to him having such a long term injury, Lambert hadn't chosen a specific number for him. Which would explain why Johnson took his, as it was never actually his.
I think when he comes back he'll get a choice over what number he can have, which will probably end up being like 26 or something.