The Belgian, who's move to United is close to be finalised, netted 25 Premier League goals for Everton last season.
But Keown claims that Lukaku is susceptible to the odd poor performance, which is something that not only Jose Mourinho, but the Red Devils playing group must help stamp out.
"If I was a player in their dressing room I'd be looking for more consistency from him," Keown said via the Express.
"Every time I've watched him live he's disappointed a little bit. I see some great performances, I see him destroying defenders, and then I see a bit of a tepid performance."
"It's an interesting one because I think if Mourinho can get him fired up for every week then he's going to be an outstanding talent but I think also the players need to challenge him," he added.