The role of motivational processes in self-regulated learning is recognized in all the models of self-regulated learning, but differently conceptualized. Integrating the presumptions of the expectancy-value model, models of self-regulating learning, and distinction between motivational beliefs related to learning process and related to self-regulation of learning, we formed the reconceptualized model of the role of motivational beliefs in self-regulated learning in the process of learning mathematics. In our model we presume that motivational beliefs related to a specific domain can be considered as a precursor of self-regulated learning, while motivational beliefs related to the self-regulation of learning can be viewed as mediators of self-regulated learning. The participants in our study were 582 secondary school students. We explored the role of their motivational beliefs related to learning mathematics (self-efficacy for learning, subjective task value) and related to self-regulation of learning (self-efficacy for self-regulated learning, perceived cost) in predicting two different outcomes of self-regulation in learning - student behavioural engagement and procrastination. The results of our research suggest that the relationship between mathematics-related motivational beliefs and criterion variables (student behavioural engagement and procrastination) is mediated by motivational beliefs related to self-regulated learning. Consistent with our reconceptualized theoretical model, our results indicate that motivational beliefs related to the specific domain of mathematics do not predict behavioural engagement and procrastination in mathematics directly as hypothesized by expectancy-value theory, while indirect effects through motivational beliefs related to self-regulation are in line with our reconceptualized theoretical model.