For the first time we explore the circumstellar effects on the Rb (and Zr) abundance determination in O-rich asymptotic giant branch (AGB) stars by considering the presence of a gaseous circumstellar envelope with a radial wind. A modified version of the spectral synthesis code Turbospectrum was used to deal with extended atmosphere models and velocity fields. The Rb and Zr abundances were determined from the resonant 7800 Å Rb I line and the 6474 Å ZrO bandhead, respectively, in five representative O-rich AGB stars with different expansion velocities and metallicities. By using our new dynamical models, the Rb I line profile (photospheric and circumstellar components) is very well reproduced. Interestingly, the derived Rb abundances are much lower (by 1-2 dex) in those O-rich AGB stars showing the higher circumstellar expansion velocities. The Zr abundances, however, remain close to the solar values. The Rb abundances and [Rb/Zr] ratios derived here significantly resolve the problem of the present mismatch between the observations of intermediate-mass (4-8 M⊙) Rb-rich AGB stars and the AGB nucleosynthesis theoretical predictions. Appendices are available in electronic form at http://www.aanda.org