Should the phrase "Man up" be considered offensive?

Liverpool Football Club have recently released a list of banned phrases, that they want the fans to avoid using. (Read the story)

One of these phrases is "Man up".

Why is this phrase considered offensive, and who is likely to take offense?

From the article:

"The club have produced a handbook which highlights common slurs against race, religion, sexual orientation, gender and disability which could cause offence."

Man up, along with a phrase such as You play like a girl, imply that it is better to be a man/male and worse to be a woman/female. Would you tell a woman to "man up?" Using a masculine descriptor as a positive or the feminine as a negative is rather insulting to females/women. Similarly, the club has banned "gay" as an insult, as implying being gay is a bad thing is offensive to actual gay people. Same thing for racial epithets, which the club has already had an issue with.

A friend of mine coined(?) the phrase "grow some ovaries," which I personally like to use.

EDIT: The complete and rather offensive list can be found here

It is likely considered offensive because it suggests that anything less than a particular version of "man" (decided by the person saying it) is inadequate, weak, etc. If a man needs to "man up", he is not a real man. The same applies to women - how can they man up? It is immediately condescending and can be considered sexist, misogynistic and prejudice because it can apply to anyone who isn't man enough in the moment, situation, group.

I personally don't find it offensive. It can be a great motivator. Political correctness...