  Recent analytical and numerical work argue that successful relativistic Fermi
acceleration requires a weak magnetization of the unshocked plasma, all the
more so at high Lorentz factors. The present paper tests this conclusion by
computing the afterglow of a gamma-ray burst outflow propagating in a
magnetized stellar wind using "ab initio" principles regarding the microphysics
of relativistic Fermi acceleration. It is shown that in magnetized
environments, one expects a drop-out in the X-ray band on sub-day scales as the
synchrotron emission of the shock heated electrons exits the frequency band. At
later times, Fermi acceleration becomes operative when the blast Lorentz factor
drops below a certain critical value, leading to the recovery of the standard
afterglow light curve. Interestingly, the observed drop-out bears resemblance
with the fast decay found in gamma-ray bursts early X-ray afterglows.
