Williams, who gave birth to a baby girl in September, has said she's planning to defend her title at the Australian Open in January –- just four-and-a-half months after becoming a mother.
But Evert said standards in women's tennis had risen this year since Williams stepped off the tour, warning that the 36-year-old would be returning to a more competitive environment.
"I think that this whole year, the level has gone up because Serena hasn't been dominant and the other players have all felt that they had a shot at it, at the number one ranking," Evert said, speaking in her role as an ambassador for this month's WTA Finals in Singapore.
"I think that because of that challenge, they have improved in the physical and the mental part of it because they've all had a shot at it and they've pushed each other and that's why there have been so many different winners. So I think the level will have improved by the time Serena comes back."
Since Williams' last match –- January's Australian Open final against her sister Venus –- Jelena Ostapenko was a surprise winner of the French Open, Garbine Muguruza triumphed at Wimbledon and Sloane Stephens capped a brilliant return from injury to lift the US Open trophy.
Angelique Kerber, Karolina Pliskova and Muguruza, the current number one, have all held the top ranking this season, and Russia's Maria Sharapova is back on the scene after returning from a 15-month drug ban.
- Asian upsets -
"There's so many things we don't know about Serena, how it's going to feel to her being a mother coming back, is it going to distract her, is it going to motivate her more?" Evert said.
"Or how being a mother emotionally will pull at her and will it take her away from tennis? There's so many elements here that we don't know about Serena and her comeback," she said, adding: "I think the other players have gotten better and I think they're not going to be as intimidated when Serena comes back."
Evert also dismissed concerns about the WTA Tour's 'Asian swing', where top players have suffered a series of upsets as the season draws to a close -- including at last week's Wuhan Open, where only one seed reached the semi-finals.
"This has been an issue that's been ongoing as long as tennis has existed, and that is the fact that it's such a physical, mental and emotional sport that by the time you get around to the end of the year... it's understandable that players are starting to feel the effects," she said.
"It has nothing to do with the Asian swing, you can find it in the men's too," Evert added. "The fact of the matter is, in the beginning of the year you're rested and sharp and it's a big year, with a lot of depth in the game and you're getting a lot more tough matches than previous eras.
"So you really have to manage your schedule so that when it comes around to the fall, you want to be feeling fit and sharp and not injured. But that's about managing the whole year, it's not just about the Asian circuit."