Sunderland have revealed Steed Malbranque's chest pains stemmed from a muscle injury.
The French midfielder was taken to hospital on the South Coast after being substituted in Tuesday's 1-1 draw with Portsmouth.
But the 30-year-old's health worries were allayed the same evening as he was released after checks and was allowed to fly back to the North East.
"Steed has a musculo-skeletal chest problem for which he is receiving treatment," Black Cats' physician Dr Glen Rae told the club's official website.
"We are very grateful for the help of Portsmouth medical team, paramedics and Portsmouth local hospital for their help on Tuesday for arranging appropriate investigations for us."
It is unknown how long Malbranque will be out of action, but the former Fulham man could be back in time for Sunderland's game with Arsenal on Saturday week.