No kidding. I'm even a bit more lenient than you guys, I rarely criticize unless they're under 6.7 or so for last five games, and I still get nothing but arguments.
And that's your problem. You pay them to play well. Players getting scores of 6.7 can be easily replaced with planted shrubs.
Tell them that they've played poorly and if they snap back, be firm! "I think you should take my criticism on board as I am the one who picks the team" and if that doesn't work, hit them with the "I am the manager and I don't appreciate being spoken to like that".
The "I am the boss...appreciate...bla" response puts them back in their place, but the issue is likely not resolved with this. You will, however, probably get a PR icon next to his name - and this is the player's way of saying he gets along with you really well.
Monitor the situation in the following match(es). If he plays poorly in the next match let him know - I find 6.8 to warrant an "you played poorly, pick yourself up" rating for key and first team players, 6.7 for everyone else. Anything under 6.7 gets an "unacceptable". 6.2-6.0 get "unacceptable" warnings, official warnings for poor performance, and 1 week fine for repeat offenders considered first team and key players. Remember, however, to always warn players for poor performance first. Never fine them straight out of the box. That never goes down well.
In the case that they complain abaout being fined, tell them that you're "sticking to your decision". In the majority of cases the player will "see it from your point of view".
In general, criticism doesn't go down too well with primadonnas or players having diffuclty adapting or players with low professional attributes. They're always proper powder kegs and should be handled as such. But always, always always remain firm. After all, you are the boss.
Also, be careful to not be an *** - players have feelings, too, and losing your temper with them when they've not warranted harsh treatment will result in an unhappy player and - depending on how popular they are in the squad - unrest in the dressing room.