In this paper, we explore the use of the standard ITU-T P.563 speech quality estimation algorithm for automatic assessment of dysarthric speech intelligibility. A linear mapping consisting of three salient P.563 internal features is proposed and shown to accurately estimate spastic dysarthric speech intelligibility. Delta-energy features are further proposed in order to characterize the atypical spectral dynamics and limited vowel space observed with spastic dysarthria. Experiments using the publicly-available Universal Access database (10 speaker patients) show that when salient delta-energy and internal P.563 features are used, correlations with subjective intelligibility ratings as high as 0.98 can be attained.