After a goalless first half, Fulham wilted at the KC Stadium after the interval and suffered their worst ever Premier League loss.
Riise echoed the apology issued to the club's fans by head coach Rene Meulensteen in the aftermath of the crushing defeat.
But the former Liverpool man reminded his colleagues to remember their recent good form, which saw them beat Norwich City at Carrow Road on Boxing Day.
"Before I talk about the game itself I'd just like to apologise to all the fans," Riise told Fulham's official website.
"They deserved more than that, we let them down. We appreciate the support that they give both at home and on our travels.
"I don't think we played particularly well in the first half, but, at the same time, I don't think Hull put us under too much pressure either.
"But what unfolded was a dumbfounding 45 minutes of football.
"They scored six goals in quick succession, and as a result we looked totally shocked.
"I don't think anyone saw that result coming and, of course, it was very difficult to take."
"This serves as a reminder of what happens when you take your foot off the gas – at this level you get punished.
"But it's important that we don't forget the good progress that we have made lately. We need to concentrate on moving out of the bottom three."
