Including Sunday’s 2-2 draw at Tottenham, the Reds will play five games within two weeks despite Covid-19 cases and injuries.
Premier League clubs chose to fulfil festive fixtures at a meeting on Monday despite a rise in Covid-19 cases.
“I don’t think people can appreciate how intense it is until you actually see it first hand,” said Henderson.
Speaking to BBC Sport, he added: “Football to us is everything and we want to be able to perform at the highest level every time we set foot on the pitch. And unfortunately, in this period it is difficult to do that.
“That has been like this for a few years now and it has been difficult but then, on top of that, you chuck in Covid and it becomes even harder and even worse.
“I am concerned that nobody really takes player welfare seriously.
“I think decisions get made – of course we want to play as footballers, we want to get out there and play – but I am worried about player welfare and I don’t think anybody does take that seriously enough, especially in this period, when Covid is here.
“We will try to have conversations in the background and try to have some sort of influence going forward, but at the minute I don’t feel the players get the respect they deserve in terms of having somebody being able to speak for them independently and having the power to say actually this isn’t right for player welfare.”
Which teams have the busiest Christmas schedule?
On Tuesday, Liverpool assistant manager Pep Lijnders said the schedule was “absurd” while Chelsea boss Thomas Tuchel said he is having to take “huge risks” with his players’ fitness.
Liverpool were missing a number of key players against Tottenham, with Virgil van Dijk, Fabinho and Thiago among those absent following positive Covid tests, while midfielder Henderson was out with a cold.
Premier League players and club staff returned a record 90 positive Covid-19 tests last week, with six out of last weekend’s 10 fixtures postponed.
“We do speak about it as players because, at the end of the day, it affects us directly,” said Henderson, who helped organise an initiative last year for Premier League players to generate funds for the NHS.
“I know people will say we do get paid a lot of money to go out and play football. I get that and understand that, but football is everything to us. And especially those players that are playing international games and European games, you get a maximum of probably two or three weeks off a year. I am not sure that is enough to physically recover and mentally recover from the season previous.
“But again, there is no communication with players in terms of what they think, which is a big problem really, I am not saying they have to make decisions on what players think, because everybody will have a different opinion, but I think they need to be part of a conversation because, ultimately, we are the ones that are going out and feeling it and playing it.”