This is because of the threat posed by his long-term rival Wout van Aert, who has looked the form rider in the Classics this season, winning at Omloop Het Nieuwsblad and the E3 Saxo Bank Classic.
Speaking to the media post-victory, the Alpecin-Fenix rider said: "I think it [his form] needs to be better."
He explained it was because of the other riders that will be present on Sunday: "Especially [Wout] van Aert, we have seen how strong he is and I've raced against him all my life so I know how my shape needs to be to follow. I think it needs to be a bit better, but I'm quite confident that with a few days of rest it will be."
Van der Poel broke away as part of a powerful group with more than 60km to go at Dwars, something he described as "too far" for the Tour of Flanders.
"Today was really hectic and dangerous with a lot of crashes," he said. "I didn't want to be involved in one of those, so we started racing sooner than we planned yesterday.
"It was impossible to come back to our group. We were seven or eight really strong riders, after Berg Ten Houte I immediately knew the race was done there."
The extra length of the Tour of Flanders is the "main issue", the Dutchman explained.
"I've done it in [Milan] San Remo but that's a totally different race so maybe it was good to do today to have a real race in the legs before Flanders but it all depends.
"In the last months I did everything I could do to be in the best possible shape on Sunday, and I think I succeeded even better than I expected. I will try to battle for the win on Sunday."
Wednesday's race was only Van der Poel's seventh this season, after a long lay off with a knee injury, something he described as "not really" affecting him at the moment.
He was not the only rider making his bow in the cobbled classics this season, as Tadej Pogačar also lined up at the race, ahead of a debut at Flanders.
The Slovenian could not follow the crucial attack though, and failed in his attempt to bridge the gap, which Van der Poel claimed to not be aware of.
"Pogačar had the legs to follow, but I think he was just a bit too far," he said. "I knew Ten Houte was a crucial point today, I was planning to attack there if it was still together, but I didn't have to cos other guys did it already.
"It was impossible to come back cos we were pacing fast right away and all the guys took their roles. It was impossible to catch us. I don't know what wrong, whether it was placing or something else, but for sure if he's in front he's in our group."
Van Aert did not race on Wednesday, but his Jumbo-Visma team still impressed, with Tiesj Benoot finishing second behind Van der Poel. Elsewhere, Christophe Laporte has also shown this season that he is a force to be reckoned with.
"It's going to be difficult," Van der Poel said. "They have three or four riders that can win the Ronde. so it's up to my teammates as well to be there in front and react to some attacks of that team."
As for Sunday, it looks like it might be a chillier edition of the Monument after a period of clement climate in northern Europe.
"I'm still hoping for good weather," the Dutchman stated. "My weather app is giving sunny and eight degrees, so I'm just going to focus on that."
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