Like the others said, liquid yeast only have sufficient cell count for beers up to approximately an og of 1.030. So you're underpitching for most beers if you don't make a starter.
The Mr Malty pitch rate calculator
is the best tool to use to determine the right amount of yeast for your job, EVEN in a 2.5 gallon batch of beer.
Even in a 2.5 gallon batch if your gravity is around 1.080, mr malty begins to suggest using a starter for a single package of liquid yeast/. It's not a lot at that point, but the calculator begins to start showing your short of cells....
So if your gravity's no more than 1.080 you should be alright with just the single tube.