An emerging research trend associating social robotics and social-cognitive psychology offers preliminary evidence that the mere presence of humanoid robots may have the same effects as human presence on human performance, provided the robots are anthropomorphized to some extent (attribution to mental states to the robot being present). However, whether these effects also depend on the evaluation potential of the robot remains unclear. Here, we investigated this critical issue in the context of the Stroop task allowing the estimation of robotic presence effects on participants' reaction times (RTs) to simple and complex stimuli. Participants performed the Stroop task twice while being randomly assigned to one of three conditions: alone then in the presence of a robot presented as competent versus incompetent on the task at hand ("evaluative" vs. "nonevaluative" robot condition), or systematically alone (control condition). Whereas the presence of the incompetent robot did not change RTs (compared to the control condition), the presence of the competent robot caused longer RTs on both types of Stroop stimuli. The robot being exactly the same in both conditions, to the notable exception of its evaluation potential, these findings indicate that the presence of humanoid robots with such a potential may divert attention away from the central task in humans.