"We think it's a delayed concussion from the tackle when he was hit behind with an elbow," Merrick told AAP in Sydney, where the rest of the team remained ahead of their clash at Allianz Stadium.
"The physio spent all night checking up on him.
"He's seeing the doctor and our specialist sports physician today,"
Merrick said the incident "doesn't look good on video" and hoped the A-League's match review panel would take the same view.
"He said his head whipped back really hard, and he hit the ground pretty hard with his face," he said.
"And I don't know why it happened because our goalkeeper took the ball easily, so there was no challenge on the ball.
"I don't know what was going on there but I was pretty disappointed. We never really noticed it where we were sitting, and he wasn't given a free-kick or anything."
Midfielder Vince Lia will plug the gap, brought in as an extra experienced head in a what will be a vital match for the Phoenix, who have slipped out of the top cluster into sixth place.
Lia was expected to be out of action for up to two months with a hamstring tear, but Merrick was positive he is up to playing as a likely substitute and could help correct Saturday's "poor midfield structure".
"We expected a 7-8 week turnaround conservatively, but he's just come on in leaps and bounds - it's been about three weeks," Merrick said.
"I don't think he's got 90 minutes in him but I'd rather him coming off the bench than bringing on a youngster in a high-pressure game like this, which I had to do in the last game."
Sydney have won their last two A-League matches against the Phoenix, though have a less-than-convincing home record and have slipped from first to fourth on the table within the space of two weeks thanks to a loss to Melbourne Victory and away draw with Perth.
- AAP
