Yes you do. There is no adequate substitute for fresh squeeze lemon juice when used in cocktails and mixed drinks. You're up the creek. Out of luck. Doomed.
Well, not doomed. It's pretty easy to keep lemons around for when you need them. Just go get some and a hand operated citrus press. The lemons will keep for a week or more in the refrigerator. You'll also be able to use the skin which is a critical part of hundreds of classic cocktails.
You will get more juice out of citrus fruit if it's at room temperature. If you know you're going to use a lot of them, take them out of the fridge about two hours ahead of time. Or to quickly bring lemons to room temperature, soak them in hot tap water for a minute or two.
The common grocery store lemon is called a Eureka lemon. This is the one you want for your drink recipes. You may also see Meyer lemons but unless the recipe specifically calls for them, steer clear. Meyer lemons are not as acidic which will throw off the balance of your cocktail recipes.
Lemon juice is a key ingredient in a multitude of cocktails, mixed drinks, and punches. Some drink categories are even based on lemon juice such as sours, fixes and crustas to name a few.
There should almost always be a sweetener to balance the acidity in lemon juice. This is usually sugar or a syrup and getting the balance right between the sweet and the sour is half the battle (or maybe even more than half) of making a great cocktail.
We've all made a drink that has been way too far over in the sour territory. You'll sometimes see 2 oz of lemon juice countered by only 1 tsp of sugar in a recipe. What's the most unbalanced drink you've ever had?