Barkley’s arrival on loan from Chelsea last summer was seen as a significant coup for Villa but the 27-year-old has struggled for consistency since returning from a hamstring injury last month.
With Grealish getting ever greater attention from opponents, Thompson believes it is increasingly important Barkey finds his form.
The England international has scored three goals in 13 appearances for Villa.
Thompson said: “He is clearly an excellent player but watching him week-to-week, he has been inconsistent.
“It is approaching the point where supporters are looking at him and asking: ‘What have you got?’
“I really do think this is the time he needs to step up and show what he can do on a more consistent basis. Villa really need him.”
Grealish is enjoying the best season of his career having already scored six times in the Premier League and set up 10 goals, for a Villa team now chasing European qualification.
But the skipper has been relatively well-shackled in recent matches against West Ham and Brighton, as opponents seek new ways to disrupt his game.
Thompson said: “Other clubs will have watched the West Ham game, when they effectively stuck two men on Jack, and saw that worked relatively well.
“It is when Jack isn’t getting much space that the onus falls on the likes of Barkley and Bertrand Traore to step into the breach and drive the attack.
“If they aren’t doing that, then quite quickly Villa start to seem a bit one-dimensional.”
Villa, who enter the weekend sitting eighth in the Premier League, are back in action on Sunday when they host Leicester.