Esmael Goncalves netted twice as the Buddies booked their spot in the last eight and Lomas said: "I felt we should have been two-nil ahead at half-time.
"Craig Samson has made one wonderful save for them and we couldn't score.
"We thought we had got over our Achilles heel in the last few games, we had looked clinical in our last two matches.
"But that's the difference and the reason as to why we are not second in the league right now. The defending for the two goals was a shambles and cost us big time."