You mean the dog lacks the characteristic black mask that should cover the muzzle up the eyebrows?
I don't think anyone could say that it's impossible for a pure bred boxer to lack a mask, but at the same time, it is not a common occurance. The breed standard describes the mask as "essential" (notwithstanding that it may be 'covered' by white in flashy individuals), and its a very typical trait of the breed (even if weaker on some individuals than others). It is not common to see a dog that really looks like a boxer be completely absent that mask.
On the other hand, there are many dogs said to be boxers that
don't look much like boxers and that also lack the black mask
Along with head shape, the mask is one of the first things to go when boxers are mixed with other breeds (breeds that don't have a black mask). Put another way, a dog without a black mask would need to look VERY much like a boxer in all other respects before I believed it was not a mixed breed.