Scarlets trailed Toulon 24-6 inside 30 minutes before losing 31-20 and face them again in Llanelli on Sunday.
"We need to start from the start, not give them a lead and then wake up," Patchell said.
"Defensively we need to sort out that first 20."
"The attacking game worked well but we need to match that with intensity in defence."
The Scarlets are in third place in the Pool 3 table with just four points after three rounds and are 10 points behind Saracens and five behind Toulon but Patchell says the Welsh region can win the reverse fixture.
"There's no reason why we can't get something out of the group," the Wales international told BBC Wales Sport.
"We're playing some really good stuff in attack and there's no reason we can't turn them over next weekend.
"Then in the last two weekends of the competition, it's all to play for.
"I think we probably gave ourselves a bit too much to do in the first 20 minutes really, they built a comfortable lead and we only got going after 25 minutes.
"When we did get going we looked dangerous and you could see they were worried about it."
Powerhouse France centre Mathieu Bastareaud, who earned the man-of-the-match award, was a key figure in the early demolition of the Scarlets defence.
"He speed-bumped me for the first try," Patchell said.
"I went in low and went as hard as I could but unfortunately when you're giving away forty kilos, there's going to be a bit of momentum.
"Bastareaud played very well and he's someone we're going to have to keep an eye on next week."
Scarlets will hope that Wales centre Jonathan Davies recovers from a hamstring injury, while international flanker Aaron Shingler should be available after paternity leave.
Scarlets fly-half Rhys Patchell says they need to start much better if they are to have any chance of upsetting three-times champions Toulon in their return European Champions Cup fixture.