I liked him against Tottenham last year in the Carling Cup - I think he is a good player in all honesty and his pace and attacking abilities are only going to make him more of a threat against teams.
Degen has got to be Glen's backup - Kelly is good but turning a centre half into a right back isnt always the best.
Like someone said above when teams are tiring - bringing Degen on will be a good option because you can either bring him on the wing with his pace or you can slot him in at right back and push Glen forward.