O'Neil set up Tuncay Sanli's winner and Southgate said: "We've missed his energy and enthusiasm.
"He's been out for two games with hamstring trouble but we needed him today with George Boateng suspended and Emanuel Pogatetz injured.
"We were down to just 16 fit players but they were resilient and had the desire and will to win.
"People probably looked at us and thought we'd be rolled over at a place like Portsmouth but I was delighted by the attitude we showed.
"We showed all the qualities we didn't show in the defeats against West Ham and Birmingham and it just shows you what you can achieve."