Newcastle United remain outside the Premier League's bottom three but the abject nature of Saturday's 3-0 defeat by fellow strugglers Brighton & Hove Albion did not bode well for their hopes of avoiding the drop.
The visitors failed to register a shot on target and are now six games without a league win, leaving them in 17th place, two points above third-from-bottom Fulham.
To rub salt into their wounds they lost midfielder Isaac Hayden to what looked a serious knee injury shortly before Trossard's opener on the stroke of half-time.
"The mood is the lowest it's been. We just weren't good enough," said Newcastle winger Ryan Fraser, who hit the post shortly before Welbeck made it 2-0.
"It was a must-win game after doing alright in the last couple of weeks. It didn't happen for us tonight.
"Conceding just before half-time after the injury was a low blow. The second goal was identical to their first. It was a little bit too easy. We need to save our season now."
While Newcastle had drawn their previous three matches, they have managed only two wins from their last 18 and with a tough run of fixtures to come after the international break, they are in a desperate battle to remain a top-flight team.
"We're not in the bottom three yet and we've still got games remaining," Fraser said. "If we go and win our next game it looks rosy again. We've still got to play Fulham too."
Bruce will come under further pressure after what he described as "a painful evening" on the south coast.
"We weren't anywhere near the levels we need to be at. I take full responsibility for that. Unfortunately it unravelled very, very quickly," he said.
"There's a lot of disappointment. Over the last few weeks I've seen a degree of togetherness and fight, but we've got to be better than we were tonight to give ourselves a chance."