The Buddies boss described his side's performance as "Jekyll-and-Hyde stuff" and expressed his disappointment with the result.
"It was Jekyll-and-Hyde stuff from us," BBC Sport quotes him as saying. "The way we took our foot off the pedal is the most disappointing thing about this defeat.
"We got off to a dream start but never took it on and I didn't expect what happened to happen.
"It means getting into the top six now is going to be a very tall order."
Despite the defeat, 11th-placed St Mirren remain 14 points ahead of Dundee in the table.