No, it does not mean that a trade is imminent, but it could be. There has been no word about a potential injury to Greinke, though it is a possibility that he is, especially given his recent performances.
Both the Texas Rangers and Los Angeles Angels have expressed interest in acquiring Greinke for the stretch run, and Brewers are quite aware of this -- and the rivalry that exists between the two clubs. Per FOX Sports' Ken Rosenthal, the Brewers are looking to potentially exploit this rivalry, and get either Texas or Los Angeles to cough up a higher price to prevent Greinke from going to the other team.
Just because Greinke is missing his next start doesn't mean a trade is imminent, but it very well could be. If he's not injured, then talks are likely pretty advanced between the Brewers and another club.
For more on the Rangers check out Lone Star Ball.