It can, but because H5N1 hasn’t yet turned into easy H2H transmission, it has forced a reevaluation of the speed of such adaptation.
The two biggest worries right now is that it may have a much milder form which encourages spread; and that they don’t yet know what its animal vectors are.
One of the factors keeping H5N1 from gaining H2H transmissibility is that humans are a dead-end host for the virus. Since its reservoir is birds, the selective pressures on the virus are for it to maintain its avian-optimized infectivity and transmissibility profile. The bigger fear with H5N1 is that it can infect a host such as swine where it would have the opportunity to swap genes with another influenza virus.
With MERS, we are somewhat handicapped in that we do not know what kinds of mutations would make it easily transmissible. At least, since its genome consists of a single RNA strand, recombination with a more transmissible coronavirus (like the one that causes common colds) is not very likely.