The 20-year-old Australia international has linked up with the Bhoys squad at their Spanish training camp on trial from Central Coast Mariners in the A-League.
And Mjallby insists there is a good chance of a deal if the midfielder shows the quality that the club believes him to possess in Tuesday's friendly against Steaua Bucharest.
"There is no point in having him here if we can't sign him right now," Mjallby told the Daily Record.
"If he is good enough why not go for it straight away?
"I don't know the figures or his contract situation but he could be a gem.
"I've heard very good reports and the sooner we see him the better.
"The reports say he's a player who should suit us as he can open up defences with his style. We haven't signed him but the good thing is he can maybe play a bit-part against Steaua."
Rogic is reportedly the subject of interest from Premier League side Reading but Mjallby only sees that as a positive.
The Swede added: "It's a good sign that a Premiership club are interested. It shows he has quality."