The Magpies were beaten 2-0 at St Mary's, the side's third Premier League defeat in a row, and they will head to Stoke on Wednesday with just a single win in their last eight matches.
This slump has left Newcastle languishing in 14th place, just four points above the drop-zone, and Pardew knows he must work hard to instil some self-belief in his out-of-form players.
When asked if he was facing his sternest test, he said: "For sure - there's no doubt about that. We are really having to fight.
"We have got players, let's be honest, who aren't playing as well as we know they can. We are struggling for a bit of confidence."
Steven Taylor is the latest addition to the club's lengthy injury list with a hamstring problem, but Pardew hopes the return of Fabricio Coloccini will ease his defensive headache.
"We are missing some key players, but one of them will be back on Wednesday night, thank goodness," he said.
"It looks like Steven's going to miss out, so it's one thing after another."