Nosworthy was a first-half substitute on Sunday as the Black Cats went down 1-0 at Fulham. The defender was still on the bench when Bobby Zamora scored the only goal of the game in the seventh minute, but even from there, it was impossible not to be alarmed at the space which was afforded to the Fulham striker as he nodded in John Pantsil's cross.
Sunderland conceded an equally soft goal to Wigan's Hugo Rodallega eight days earlier, and with Saturday's home game with Portsmouth having assumed increased importance on the back of two successive defeats, Nosworthy admits his side's defending will have to improve this weekend.
If we don't concede, it makes it easier to go up the other end and nick something,
I can't really put it into words, but we're just not performing at the moment. It's not as if we can't do it, we're just losing our way a little. We need to dig deep and work hard on changing things around with a win.
In recent weeks Sunderland have struggled at both ends of the field, with an inability to keep clean sheets being exacerbated by a failure to convert opportunities of their own.
Darren Bent's winner against Arsenal is their only goal in their last four league games, and the Black Cats have become hugely reliant on the form of their two centre- forwards.
Of the 18 Premier League goals scored by Sunderland players this season, only four have been claimed by someone other than Bent or Kenwyne Jones, and Nosworthy admits such a situation is unsustainable.
We can't just rely on Bent and Jones to score, he said.
We need the breaks, and we need to start putting away those little half-chances.
Meanwhile, Jean-Yves M'Voto has cut short his loan spell at Southend United in order to complete his rehabilitation from a hip injury with Sunderland.
Sunderland will face Barrow in the third round of the FA Cup after the Conference National outfit beat Oxford United 3-1 in their second round replay last night. The game will be at The Stadium of Light on January 2.
Ryan Noble was the four goal hero as Sunderland stunned hosts Sheffield Wednesday 7-2 to secure a place in the fourth round of the FA Youth Cup. Matthew Fletcher, Michael Lamb and Craig Lynch also scored.