Hi. My Fiancé Came Off His Bicycle And Badly Grazed

Hi. My fiancé came off his bicycle and badly grazed his knee about 6 weeks ago. The healing process took a while but eventually the scab disappeared and a 'new, fresh, pink' scar remained. After 2 weeks off a clear knee cap he played football and that evening noticed a blister had appeared. It seems to be full of clear fluid but it is very red! It is probably about 2cms across. Does this require medical attention?

Thank you