I don't see those lands becoming Catholic on their own, but major parts of both Belarus and Ukraine were ruled by Lithuania and later Poland-Lithuania. If the Slavic lands east of them remain disunited and Moscow (or any other contender for unifying the Greater Russians like Tver or Novgorod) isn't able to beat back the Catholic Poles/Lithuanians, then those areas might become more and more Catholicized, but probably not until the late 1600s and 1700s.
They might retain much of their Orthodoxy, but still be in communion with Rome - a larger uniate/Greek Catholic Church. IOTL, resentment against perceived Polonization/Catholicization lead to the Cossack Rebellion against the Poles in 1648. Without a powerful Muscovy, conditions might change so that such a revolt never takes place, or at least is not successful.