McGugan scored in the second half against his former club to secure a 1-1 draw for the Hornets, ending Forest's 100% record in the process.
Zola admitted that the 24-year-old was not at his best in the first half but believes that he coped well with the pressure of playing against a club that he spent seven years with.
"Sometimes it is the way, you can try to do too much against your old team or you can really be up for it, and I thought Lewis was in control," Zola told the Press Association.
"In the first half he was not at his best but he waited and produced a bit of magic which was welcomed by all of us. I see a lot of potential in Lewis. I'm expecting more than that. It's not just about scoring goals, I know he can do that, it's about being more effective on the pitch and defending as well.
"There were other teams interested in him, he's a quality player, but he was very happy to come here."
Watford now sit fifth in the Championship table, having picked up two wins and two draws in their opening four games.