Dundee United are awaiting news on the severity of Benjamin Siegrist's injury after a training-ground issue kept him out of a 1-0 Tayside derby victory against St Johnstone.

Former Motherwell goalkeeper Trevor Carson made his debut after Siegrist suffered a knee injury just ahead of the final week of the transfer window.

When asked about Siegrist's injury, United head coach Tam Courts said: "We're not entirely sure, but in training with nobody near him he picked up an injury which will keep him out longer than just days, shall we say.

"We don't know the diagnosis yet but he'll be sent for a scan."

Northern Ireland international Carson made a good save from Glenn Middleton towards the end of an assured debut.

"We knew what we were getting with Trevor," Courts said. "He's an international goalie and he's been a dream to work with. We knew if he was in the team he'd be more than capable."

United won thanks to Peter Pawlett's goal on the hour mark despite losing their match-winner to a second yellow card in the 78th minute.

Pawlett had been booked for celebrating his goal with some of the 3,000 travelling fans and was then ruled to have dived under the attention of Murray Davidson in the centre circle.

"The first was very harsh for an over-exuberant celebration. I didn't see it like that so I was very frustrated," Courts said.

"And the second I had quite a good view because it was almost in line with the dugout.

"Peter's known to be a very quick dribbler. He ran across the St Johnstone player and there appeared a minimal touch which is all you need when you are running at that speed. It seemed to be a very harsh decision."

United can appeal the second yellow and Courts said: "Once we review it we'll decide. From my perspective there seemed to be a touch but you always reserve the right to go back and make sure we choose the best course of action.

"It was a really big moment, it was a counter-attack situation and the pitch started to open up for us. I don't think Peter would have been looking to go down at that point."

St Johnstone manager Callum Davidson admitted his team were below par as they juggle European and domestic duties.

Davidson was without defender Liam Gordon but only made one other change following Thursday's 1-1 draw against LASK in Austria with Michael O'Halloran dropping to the bench.

"For me we weren't quite sharp enough," he said. "We weren't reacting quickly enough.

"Whether you put it down to Thursday or not, as a club we have to deal with how do we get the best out of the players in big games because we weren't quite at it.

"A draw was probably a fair result. We gave away a sloppy goal. I don't mind if they score good goals but we could do much better for their goal, then we didn't have enough zip up front."

Davidson added: "Gordon had an injury and wasn't one we could risk. Hopefully it'll have settled down in time for Thursday."