First language learners use various linguistic cues to construct their language(s)’ grammar. I study how phonological and morphological structures interact in the minds of two young learners of Russian (1;6 - 2;11 years of age) and their caretakers. While phonology is traditionally recognized to develop ahead of morphology, my data show a ‘backward’ directionality for the younger child only: the grasp of consonant cluster inventory depends on the rising morphological complexity, measured as a number of morphemes per word. This interaction is first backed by consonant clustering, being a natural result of phonological alternations at the morpheme boundary; and second, by language pathology data, an aphasic patient attempting to improve the coda sonority in inflected words but not monomorphemic ones during speech production. If this result is validated on a larger sample, the finding is meaningful both theoretically and practically for the speech therapy procedures.