The declining lightcurve of the optical afterglow of gamma-ray burst GRB000301C showed rapid variability with one particularly bright feature at about t - t _ { 0 } =3.8 days .
This event was interpreted as gravitational microlensing by Garnavich , Loeb & Stanek ( 2000 ) and subsequently used to derive constraints on the structure of the GRB optical afterglow .
In this paper , we use these structural parameters to calculate the probability of such a microlensing event in a realistic scenario , where all compact objects in the universe are associated with observable galaxies .
For GRB000301C at a redshift of z =2.04 , the a posteriori probability for a microlensing event with an amplitude of \Delta m \geq 0.95 mag ( as observed ) is 0.7 % ( 2.7 % ) for the most plausible scenario of a flat \Lambda -dominated FRW universe with \Omega _ { m } =0.3 and a fraction f _ { * } =0.2 ( 1.0 ) of dark-matter in the form of compact objects .
If we lower the magnification threshold to \Delta m \geq 0.10 mag , the probabilities for microlensing events of GRB afterglows increase to 17 % ( 57 % ) .
We emphasise that this low probability for a microlensing signature of almost a magnitude does not exclude that the observed event in the afterglow lightcurve of GRB000301C was caused by microlensing , especially in light of the fact that a galaxy was found within 2 arcsec from the GRB .
In that case , however , a more robust upper limit on the a posteriori probability of \approx 5 % is found .
It does show , however , that it will not be easy to create a large sample of strong GRB afterglow microlensing events for statistical studies of their physical conditions on micro-arcsec scales .