The rock-bottom Baggies - who could have climbed above Newcastle with victory - slumped at The Hawthorns as a blunder from Leon Barnett allowed Damien Duff to open the scoring in the second minute.
Marc-Antoine Fortune equalised almost immediately, but then Abdoulaye Meite failed to clear in the ninth minute and Peter Lovenkrands restored Newcastle's lead.
Steven Taylor extended the advantage before the break after losing Meite at a corner and Mowbray - who only saw Fortune net a consolation in the second half - admits his backline needs to improve.
"To basically shoot ourselves in the foot twice in the opening 10 minutes was always going to make it a difficult day," Mowbray told Sky Sports.
Mountain"And conceding a very basic goal from a set play before half-time gave us a mountain to climb that we could not overcome."
And Mowbray - who is now planning to focus upon defensive training with his side before travelling to Fulham on 22nd February - fears Barnett and Meite were struggling with a lack of match fitness.
"In defence of the players who made the mistakes, neither of them have done much training in the past month," added Mowbray.
"Both Leon and Abdoulaye have been injured in the last month and have not done much training.
"We have got a fortnight now until the next game and we have got to get lots of defensive work into the team.
"We have got to improve in that department if we are going to give ourselves a chance of staying up."