Boss Benitez said: "I don't want to say this as an excuse. United don't have another competition, they don't have the Champions League.
"In the league they are 15 points ahead so their priority is the FA Cup.
"We consider all these factors. We have to put out a competitive team that can beat Southampton. But if I don't pick, for example, Ashley Cole, if we lose, people will say, 'Why did you pick Ryan Bertrand instead of Cole?'
"We have a lot of information and it's not easy to make a decision. It's impossible to be fully fit.
"All of them can play but at which level? Sometimes you're not playing 100 per cent and still you can win."