Grix’s troops went down 44-34 to Toulouse last weekend after conceding 34 unanswered points within the first half an hour.
Despite fighting back, Panthers fell short and Grix said: “There are enough encouraging signs, but still a lot more holes that need fixing.
“It’s all well and good showing that tenacity and intensity when you’re chasing, but we need that from minute one in future.
“There’s no point giving all you’ve got when you’re behind, as we won’t win many games like that.
“It has to be defence that we work on this week. I have no doubt in our ability to go forward and score points.
“Our defence needs to be good. If we give Oldham the ball for 10 or 12 minutes and they put us in a bad position, it’ll be a really hard match.
“It is up to us to get ourselves in order.”
Panthers centre Greg Worthington added: “We have to be on the ball from minute one. We cannot give away that much territory and field position at any level.
“There has to be that desire straight away, or we will be finding ourselves on the back foot again like we did last week.”
Oldham were beaten 38-24 by London Broncos last weekend, and Worthington expects a tough test against a physical and skilful side.
He said: “We have to go there and expect to face the best version of Oldham. We have to take a look in the mirror and look to rectify mistakes this week.
“Rugby’s a great sport, as we have the full week to fix the issues in our side. We have to be at our best and string our qualities together.”