The on-loan Seattle Sounders striker has struggled to make an impact since his arrival.
Riise said: "People say when you go to America you are on your way down but you can see he is still on top of his game in training.
"He knew he was going to come back to the Premier League on loan so he has stayed fit. He did very well when he was here before, and then at Tottenham, but he is really keen to prove that he is still more than good enough. He is aggressive and knows the Premier League so he has given us a boost."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball