The 26-year old required a stretcher to leave the pitch at McDiarmid Park after coming on as second-half substitute in the 1-0 loss to Hibernian and will now be sidelined for some time.
"I've been up to see him this morning and he had already seen the specialist who unfortunately has confirmed that it is a break. He'll be operated on this (Thursday) afternoon,” Lomas told the club’s official website.
"I'm gutted for him. Robbo's a great guy to have around the place and I felt he was showing signs in recent weeks of being able to make a bigger contribution to the team.
"We wish him well and I know the lads will rally round him."