Carroll, 28, went off injured in the closing moments of an epic tie, in which his side came back from 4-1 down before losing on penalties.

It means Robinson is now without seven players from his first-team squad.

"Jake has got a serious injury. It doesn't look good at all," he said.

"We're down to bare bones now."

Jon Obika's brace either side of Liam Polworth's leveller, an own goal, and Sam Foley's strike had St Mirren in control of the contest at the break.

But Tony Watt and Rolando Aarons reignited the tie, before Allan Campbell's cross deflected in to level. However, neither side could score in extra time and St Mirren went on to win the shootout.

The dramatic defeat means that Motherwell have now gone six games in all competitions without a win, albeit they remain level on points with third-place Aberdeen in the Premiership.

Robinson's side travel to face local rivals Hamilton Academical on Saturday, with the manager vowing to make changes if he can.

"People were off form and weren't playing at the top of their game," he said of the cup defeat.

"I've been very loyal and stuck with boys that were very, very good for me in the first half of the season and they haven't been the same in the second half. So we will have to make changes.

"If we put on that second half and extra-time performance then we'll be okay and we'll stay where we are. But we need to turn our form around very quickly."