Lopez scored four minutes before the interval and twice after the break, with Lucas Akins completing a comprehensive 4-0 win for the hosts.
Spanish striker Lopez, who began his career with Atletico Madrid playing in the same youth team as Fernando Torres, was only recalled from a loan spell at Barnet in midweek.
Smith said: "It's a wonderful result. I'm sure that Dani Lopez will make the headlines and rightly so. His performance was the highlight of the season so far.
"He did great for the first goal, I thought the defender was climbing all over him and he used his body well and turned him nicely to score. If we all get the opportunity to see him enough, when he gets in front of goal you feel confident that he will at least make the keeper work, while his third was an absolute corker.
"He has been out to Aldershot and Barnet in League Two on loan, he speaks very little English, he's had to find out about the English game the hard way and take a few knocks.
"Sometimes it doesn't matter what level you're playing at, you feel like you will score goals. He has got bundles of ability and two very good feet and if you can get him in around that area, he will work an angle and get a shot off."
United boss Danny Wilson said: "I think we have made all of our mistakes this season in one game. Defensively we were second best, we have allowed too much space for all four goals, and they were poor, poor goals.
"Throughout the game I thought we were second best in all areas. It shouldn't be a shock because results like this can happen.
"It's how you respond that is the best tonic. But in saying that we have got to do better in all areas, there were too many parts of our performance where we weren't competing, where we have been previously."