The frontal aslant tract (FAT) is a recently described major connection between the preSMA and Broca's area, whose functional role remains undefined. In this study we examined a patient presenting a morphological overregularization strategy in a verb generation task during awake surgery. This specific language deficit coincided with brain tumor resection at the level of the left FAT. During the task execution the patient formed the non-existent verbs by applying a morphological derivation rule to the given nouns, instead of retrieving the appropriate verbs. DTI results confirmed left FAT damage. Neuropsychological follow-up showed that this morphological derivation impairment partially persisted after surgery, whereas the results on a wide spectrum of other language-related tasks remained satisfactory. Additionally, we compared the pre- and the post-operational fMRI activation maps for the same verb generation task. We discuss the potential role of the left FAT in the morphological derivation process and in lexical retrieval.