Employers assessing whether an employee is protected as disabled before deciding on a course of action will not be judged with the benefit of hindsight. Whether an impairment which has ceased is likely to recur (and therefore satisfy the definition of disability) must be judged at the time of the alleged discrimination.
The Court of Appeal has sensibly clarified that a tribunal cannot take into account events that post-date this, a point that has given rise to conflicting decisions at EAT level. Of course, where a prognosis is uncertain (as will often be the case), employers should err on the side of caution and assume that the employee is protected. (Richmond Adult Community College v McDougall, CA).