United leapfrogged Ross County in the Clydesdale Bank Premier League and have moved level on points with Kilmarnock in fifth place courtesy of Johnny Russell's first-half strike.
"It's a massive three points especially after Saturday," said McNamara.
"Motherwell have some good players who can hurt you.
"However, our keeper didn't have many saves to make while theirs was man of the match.
"I thought it was a great team performance, we passed up a few good chances before McFadden came on and I must admit I feared the worst.
"I said to the lads before the game that clean sheets will win you games, we have to make sure we defend well as a team.
"Some of the players we have at our disposal are fantastic talents, guys like Russell and MacKay-Steven were excellent."