Iceland under-21 international Finnbogason and Argentine Gimenez both played as the Robins began their preparations for the forthcoming Championship campaign with a 3-3 pre-season friendly draw against Skonto Riga in Latvia.
But, Johnson has not been impressed with what he has seen so far and is demanding more from the pair.
"I have not seen enough of Finnbogason yet but he did not do enough. Gimenez also did not do enough," Johnson told the club's official website.
"The two trialists out there tried a few things but I think it affected us a little.
"It is a good lesson for them but it is only their first game."