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The Sky Blues sit in fourth spot on the ladder after last Saturday's 3-3 draw with Melbourne Victory, a game that saw Terry Antonis require 10 stitches in his forehead after a flying elbow by Mark Milligan.
The incident went unpunished on the field but Milligan was subsequently handed a three-game suspension by the match review committee on Monday.
Arnold refused to say what he thought about the ban but the the club is privately fuming that Milligan wasn't even booked after referee Peter Green missed the incident.
The ex-Socceroos boss also hinted that other teams are targeting Sydney's flair players with overly-physical tactics.
Iraqi winger Ali Abbas is sidelined for the year with a ruptured anterior cruciate ligament after a late tackle by Western Sydney Wanderers midfielder Iacopo La Rocca.
Abbas, along with striker Corey Gameiro, is one of three players out with the same injury, with Nick Carle set to undergo surgery next week after coming off second-best in a tackle late in the game against Victory.
"I don't think I've had one ACL in 15 years of coaching, let alone three in four weeks," Arnold said.
"We got beat up physically by Melbourne Victory and we have to probably toughen up ourselves and fight the fight.
"Technically we have some quality footballers but the way the A-League is going at the moment, we need to muscle up."
The injuries to Abbas, Carle and Gameiro will allow Arnold to bring in injury replacement players in the January transfer window.
He has made no secret of his desire to add some pace to his side but a player who can deliver a strong physical presence in midfield is now also on his wanted list.
"We could do with a couple of players coming in who can add a bit of physicality to our game," he said.
"We're no closer to bringing anyone in, we're getting phone calls, emails put forward, but we only have a certain amount to spend with injury replacement money.
"Good players don't come cheap and if I have to, I'll give Australian kids a chance."