Dean Smith's side survived by the skin of their teeth last season, but with Grealish leading the charge, they have established themselves as a Premier League outfit this time around.
The 25-year-old has scored six goals and set up 12 more in the current top-flight campaign, but they have struggled for consistency during his injury layoff, and Merson believes that Villa will be at the wrong end of the table if their talisman ends up leaving this summer.
Speaking to Sky Sports News, Merson said: "I said it when he first got injured: Villa are a completely different team.
"They're solid enough at the back, they've got two good centre-halves, the two full-backs are decent and the goalie is good. But they're not going to score any goals without Grealish. You wouldn't back Jack to get 20 goals a season, but he does create a lot of chances and he's their go-to man.
"Without being horrible, they're a relegation team without Grealish in the team. Not this season, but if Jack ended up leaving, you'd expect them to be well down in the bottom half of the table."