Liverpool's squad is small by design isn't it? Thought Klopp likes to keep the same team more or less and doesn't change it up much unless forced. They play a well drilled system so it's not the end of the world if a player needs to be replaced in the XI as long as they can do a decent job, it also helps that they don't really take cup games seriously.
I wouldn't use united as an example, you're basically as good as us at squad building albeit with deeper pockets. Compared to City and Chelsea yeah their bench is lacking but it hasn't stopped them competing so it's a bit of a non issue.
I'd only take Martial over Firmino due to age/value, if they were the same age it's Firmino every time.