As i said there will be a de-brief. I wasn't party to the plans neither were any of us. It went ary but in my opinion and from what I saw the police's tactics were to ensure the parties were kept apart.It doesn't take a lot of hindsight to know that it was a high profile game that would be well attended. The club hire and pay for stewards and the police so I am assuming they were in the vicinity already, although this could be wrong and they may have been called for back up when things escalated.
If in the vicinity and with a giant killing act in the offing they had ten minutes to put some safeguards in place, again that isn't hindsight it is common sense and doing their job.