Western Sydney coach Alen Stajcic has blasted criticism surrounding his decision to bench high-profile A-League Men recruit Juan Mata as "cowardly".

Backing his choice to start the Spaniard on the bench for the Sydney derby, Stajcic slammed Mata's player agent in the aftermath of their 4-2 Unite Round loss at Allianz Stadium on Saturday.

The new Wanderers coach had drawn the ire of agent Fahid Ben Khalfallah after he benched the former Manchester United star at halftime during their round four 4-3 loss to Adelaide United.

Mata's arrival to Western Sydney had generated massive hype, given his brilliant career included winning the 2010 World Cup and 2012 Euros with Spain.

"I think it's just insane," former Melbourne Victory and Brisbane Roar player Khalfallah told the A-League Off Air podcast before their Unite Round fixture.

"When you bring someone like him, you have to play him in a system that suits him.

"I think it's disrespectful. I hate it. I really, really hate it because it makes no sense to me."

Mata did not start in the Sydney derby, instead joining the fray in the 57th minute.

Asked about why he had started the 36-year-old on the bench, Stajcic said: "His agent doesn't control the team.

"Juan's come to the team, he's humble.

"I've got 12 guys who don't play every game. I don't get phone calls from every single agent whose player is not playing.

"I think he's (Khalfallah) got a bit too much oxygen, to be honest. Fahid and his comments were out of order.

"They were cowardly."

Mata's fitness levels have been a talking point since the Wanderers announced the signing in early September.

He had not played competitive football in a year when he joined Western Sydney on a one-year deal, last featuring for 10 minutes off the bench for J1 League team Vissel Kobe in September 2023.

The midfielder's last start across any league or tournament - before arriving at the ALM - was in March that year when he was playing for Turkish club Galatasaray.

But Stajcic dismissed suggestions Mata had lacked fitness when he was benched in their Adelaide game and reiterated how important the Spanish star was to the Wanderers after their Sydney loss.

"Juan came on and had some good moments. It was a tough game to come into," Stajcic said.

"He controlled the game a little bit, had some moments where he could have done a little bit better as well.

"He's an important player for us at the moment. There were moments of the game where we needed him there, and he had some good actions.

"It's up to him to be able to contribute when he does come on."

Sydney's rival marquee signing Douglas Costa returned to the pitch for their second-ever ALM game after picking up a hamstring injury.

Costa was substituted out at half-time but Stajcic's counterpart Ufuk Talay said the move was to prevent further injury.

"He was always going to play 45 minutes, and it's great that he got the minutes in his legs," Talay said.