Malbranque dropped to the turf holding his chest just before half-time at Fratton Park before walking off the pitch.
But manager Steve Bruce has confirmed that Malbranque was taken to hospital by paramedics for assessment.
It's a strange one, said the Sunderland boss. He got a pain in his chest but he didn't get a whack or anything so we're not too sure. He's away to hospital, I believe, so we'll see how he is.
There was no further news on the situation, with Bruce and the Sunderland squad flying back to the North-East from Southampton at 11.30pm.