However the tables were turned when centre Mafi was sent to the sin-bin on 51 minutes and was followed there by flanker Peter O'Mahony and replacement Damien Varley.
The Dragons only had two Jason Tovey penalties to show for their numerical advantage but McGahan warned they need to tighten up their discipline for the remainder of the RaboDirect Pro12 season.
He said: "Three in the first game is something we really need to look at. Thirty minutes without your full complement makes it difficult.
"At 17-6 at half-time, we really needed to continue the momentum, but they hung in well and losing three men certainly puts the brakes on that.
"But we showed some endeavour and had some nice offloading there and made some incisions, but we just were not clinical enough to get it over, and our reaction time on a few things was off."
However there were positives for McGahan to take from the game.
Despite his yellow card, O'Mahony had a bright game vindicating McGahan's decision to give him the captaincy in the absence of Paul O'Connell.
And McGahan believes he is only going to get better as the season goes on.
"Peter was very good again. He's a young man who suffered with injuries for quite a while which disrupted his progress.
"At the end of last season though he worked his way in to the match 23 for the Heineken Cup and Magners and hopefully he'll now go from strength to strength.
"He's got great leadership abilities, and playing with James Coughlan in the backrow is a perfect opportunity for them to develop a leadership platform for the future."
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