Rhys Priestland’s last-gasp penalty consigned Mallinder’s men to a 32-30 loss at The Rec on Friday night.
It was a gut-wrenching reverse for Saints, who had led on two occasions late in the game.
And they must now bounce back at Kingston Park on Sunday as they aim to stop Newcastle Falcons claiming a league double over them.
“We keep saying it’s a season-defining period,” said Mallinder, whose side now sit seventh in the Aviva Premiership standings.
“We wanted four points at Bath on Friday night, but we haven’t got that so we’ve got to move on very quickly.
“We’ve got a trip up to Newcastle this week, which will be tough.
“But we’ll prepare properly and hopefully we can get a win up there.”