Bournemouth manager Eddie Howe expressed frustration after seeing midfielder Andrew Surman forced out of the clash with Middlesborough over the weekend with a hamstring injury.
To add insult to injury, the Cherries lost the fixture at the Riverside Stadium 2-0, with the goals being supplied by Gaston Ramirez and Stewart Downing.
"When you see players go off with hamstring injuries you are looking at weeks rather than days," said Howe.
"He's been so consistent for us and he hasn't picked up any niggles or problems, so it's come out of the blue.
"We'll assess the damage and hopefully it won't be too long. He's been magnificent for us for a long period of time. He is a huge player in our team and we are going to miss him for however long he is out."
Surman had missed only one league game this season and has been one of the stand-out players for the south coast club.
Bournemouth must wait to find out the extent of their key man's injury, but they could be looking at an extended period without him.
Sunderland will make the long trip down from the north east to face Bournemouth next weekend with the Black Cats eager to turn their season around having made their worst ever start to a Premier League campaign.