Cancer battler Bradley Lowery has been suffering "horrendous pain" as doctors fear the disease is progressing.
Five-year-old Bradley was due to be a mascot for Sunderland's match against Middlesbrough on Wednesday evening but was not well enough to attend.
The youngster, who has a rare neuroblastoma cancer, has touched the world of football through his battle to overcome the condition - and made close friends with England forward Jermaine Defoe along the way.
A post on Facebook on his campaign page, apparently written by his mother Gemma, announced that Bradley had been taken to hospital.
It read: "Bradley is not good, he has been in horrendous pain with his leg since yesterday to the point he couldn't move in bed last night.
"We have had him to the hospital today and they have given him an X-ray but it didn't show anything.
"The doctors' gut feeling is that it is his cancer progressing but we need to wait and watch for a few days to see what happens.
"I am hoping and praying that it is nothing serious and he starts to improve as I'm not ready for this to happen yet."
Last month, Bradley got a rousing reception from England fans at Wembley as he walked out as a mascot during the World Cup qualifier against Lithuania.