Froome won his fourth Tour title last July and backed it up with the Vuelta a España title, becoming the first to win the Tour/Vuelta double since the race was moved to its late summer date in the 1990s.
The Giro d'Italia crossed his mind, becoming one of the few riders to win all three Grand Tours, but Poels, who just completed the Tour of Guangxi in China, said Froome should focus on the Tour.
"I don't know, but if I was Chris I'd go of the Tour because if you can win the Tour five times then... He's already a legend, but he'll be in a really elite group if he wins five," Poels told Cycling Weekly.
Jacques Anquetil, Eddy Merckx, Bernard Hinault and Miguel Indurain hold the all-time win record in the Tour with five titles each. Lance Armstrong won seven straight but officials stripped his title when they discovered he had doped.
Froome is the only cyclist to win four titles. This summer, he surpassed Greg LeMond, Philippe Thys and Louison Bobet with three titles.
"I think it's always harder to win, the rivals are always raising their game, but he won the Tour and the Vuelta this year, so why can't he win the in sixth months the Tour again?" Poels said.
"I never hear him talk about [the five-time winners] but I think that he's thinking about it. It's quite normal that if you are so close you have to go for that one."
The Dutchman joined Team Sky in 2015 and became one of Froome's star helpers. He supported him to his 2015 and 2016 Tour titles. On his way, he won Liège-Bastogne-Liège and the Volta a Valenciana overall.
Watch: Tour de France 2018 route guide
Sky overlooked him in its Tour team in 2017 due to knee problems, but took him to the Vuelta. Poels plans to make sure he is back on track for 2018 and Froome potentially historic run.
"I hope that we have a little bit better luck next year, but it should be OK. This year, I moved to Monaco, so it was different. In The Netherlands, I'd started training on the flats, a little bit more easily, but this time it was straight away into the mountains and a little bit of a few things.
"I always have personal goals, and I think I want to go to the Tour again, so I'll sit this winter with the team and make a nice plan and see what the programme will be.
"I know on my own since I've been a professional for a long time that you have to work hard, you have to have a consistent level to the Tour. Especially in this team, you have many other riders who can replace you."
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