According to Alex Marvez of FOX Sports:
Harrell was the backup quarterback for the Green Bay Packers that lost out to Vince Young. Harrell isn't a particularly good quarterback, but it really doesn't matter. There is almost zero percent chance that Harrell actually makes the roster. More likely, this means that Greg McElroy, who has been dealing with an ankle injury, will not play this Thursday against the Philadelphia Eagles. Instead of forcing Matt Simms to play all four quarters, Harrell will likely spell Simms so the team has a backup option, especially in case something happens to Simms. I really wouldn't read too much into this.