The onset of thermalization in heavy ion collisions in the weak coupling framework can be viewed as a transition from the initial state Color Glass Condensate dynamics, characterized by the energy density scaling like $\epsilon \sim 1/\tau$ with $\tau$ the proper time, to the hydrodynamics-driven expansion of the quark-gluon plasma with $\epsilon \sim 1/\tau^{4/3}$. We argue that, at any order of the perturbative expansion in the QCD coupling constant, the gluon field generated in an ultrarelativistic heavy ion collision leads to energy density scaling as $\epsilon \sim 1/\tau$ for late times $\tau \gg 1/Q_s$. Therefore it is likely that thermalization and hydrodynamic description of the gluon system produced in heavy ion collisions can not result from QCD diagrams. Thus, it appears that the apparent thermalization of quarks and gluons, leading to success of Bjorken hydrodynamics in describing heavy ion collisions at RHIC, can only be attributed to the non-perturbative QCD effects.