I think when they are talking about how physical it is, it is not so much the agression/fouls, but more the speed and the amount of effort it takes to be competitive. I am sure that if you looked at someone like Rojas in the A League, he would cover (say) 7 or 8 kms in a game, where the expectation would be in Germany that he would be covering around 10 or 11 kms. There will be less time to recover during games, and there will be a period of adjustment to get to that fitness level.
Having said that, loaning him back to the league he has just come from will not assist him in building to that level in any way.
has anyone actually gone straight from the A-League to being a starter in a top 5 European league? I don't think anyone has, and it's probably unrealistic to expect Rojas to either for exactly this reason. It's just a huge step up. I don't think he's a Bundesliga player just yet, but they probably already know that as well
The difference in Europe is all round strength. Some A-League players will be strong but not quick, they might have good straight line speed but not turn quickly - he's been able to target that. You may be able to get away with bad tackles in the A-League but that's not what makes a league tough. It will be a different ball game in Bundesliga - defenders at that level have it all, quick, athletic, technically good and strong. He'll need to adjust his game. It's a really exciting move though!