Jurgen Klopp called on officials to do more to protect players after Liverpool's win over Burnley, suggesting some Premier League rule changes are more suited to "wrestling".
Klopp watched his side make it two wins out of two at the start of the new Premier League season as Diogo Jota and Sadio Mane sealed a 2-0 victory over the Clarets.
The Premier League's refereeing chief, Mike Riley, said there would be a move towards "letting the game flow" and not penalising "trivial offences" in 2021/22.
But the Liverpool boss was unhappy with a number of Burnley challenges he deemed overly physical during the game at Anfield and hit out at rule changes which he said feel "like we're going backwards".
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"We always had to be ready for a proper fight and we were," Klopp told BT Sport.
"You saw these challenges with [Ashley] Barnes and [Chris] Wood and Virgil [van Dijk] and Joel [Matip].
"I'm not 100 per cent sure if [officials] are going in the right direction with these decisions. It's like we're going 10 to 15 years backwards. The rules are like they are, but you cannot defend these situations. That's how it makes the game really tricky.
"I don't think it's right, but I cannot decide these things. I heard about leaving the game [to] flow, but now we have these situations. The second goal from Brentford [against Arsenal on the opening day] must be a foul - you cannot clip the arm of the goalkeeper and say, 'that's football'. I think maybe we have to think about it a second or a third time.
"The message now is let the game flow, but nobody exactly knows what that means.
"I like decisions that favour the offensive team, that's fine. But we have to stick to protecting the players. We cannot deny that. If you like that sort of thing, watch wrestling.
"In a lot of moments, we played a brilliant game without scoring. In all the other moments we had to fight incredibly hard. We won it and nobody got injured."
Asked later in his press conference about the changes he would like to see in terms of officiating, Klopp said: "I am not the Pope of football or whatever or any kind of priest who wants to tell the people what to do, really not.
"When we started to talk about the elbow in the challenge, challenges in the air, we heard the whistle. It happened then that players started feeling something in the face even when nothing touched them because it helped always to do this… now there is a message, like a headline, 'let the game flow'.
"It started with the penalties, 'no soft penalties anymore', which is absolutely fine. But we cannot forget that we have to protect the players as well and the challenges in the air… I don't say they are all fouls, for sure not, most of the time it is a clear challenge but if you do these kind of things constantly…
"There are different ways to play football, obviously, that's all fine, there were just a few harsh situations. We are early in the season and I don't want to open already the box of Pandora, I just think we have to speak about these kind of things because the intensity of the season didn't even start."
Burnley boss Sean Dyche was disgruntled himself with at least one decision, feeling Dwight McNeil should have had a penalty when he was bundled over in the box in the first half.
"We created chances, as well as defending, and could/should have had a penalty in my opinion," he said.
More from Klopp after Burnley win: Fans, Van Dijk, Alexander-Arnold and Elliott
On Harvey Elliott, making his full Premier League debut:
"Harvey was part of this performance.
"Everybody wants to talk to me about Harvey and I understand absolutely - when an 18-year-old boy plays such a mature game I can understand why everyone was asking, but I was not surprised he played like this.
"That is exactly how he has trained now for six or seven weeks since we are back, since he is back from loan."
On the impact of Virgil van Dijk's return:
"The package of Virgil van Dijk is really helpful. I like talking about Virgil but I don't like when we talk about last season [that it's about] 'Virgil van Dijk didn't play'.
"Just to mention again, Joe Gomez and Joel Matip didn't play as well which didn't make the situation easier and people tend to forget how good they are.
"But of course it's cool having Virgil back.
"He was for sure in moments on the edge today, not physically, not in a dangerous way; just the intensity of the game was incredible - especially for the centre-halves.
"You are constantly in the air, the ref whistles absolutely nothing, strikers are in your body. That was a proper test today and, honestly, I love football, I love all these challenges, all these kind of things, yes, but we came from protecting the players slightly more to letting the game all flow. There is a grey area in between which for sure we will have to adapt again during the season because just now letting the game run doesn't sound like it makes too much sense."
On Trent Alexander-Arnold and Kostas Tsimikas:
"Both full-backs had a good game. Both spectacular, if you want, with two nice [assists] - Kostas with the cross and Trent with the little cheeky curve ball or whatever for Sadio.
"They were defensively really good as well in a really difficult game from a defensive point of view."
On returning fans to Anfield:
"Everybody was really looking forward to this football festival, to this game, and I think nobody leaves this place today with any kind of disappointment because I think all of our dreams were fulfilled today, atmosphere-wise.
"That was, for sure, the best 12.30pm atmosphere we had. It was pretty special, really special."