Yes,it'll decrease his attacking performance if a winger is dml dmr too. Here are pics of my old winger who had no dmr role ,compared to my new guy with dmr.they both have similar attacking % and physical but new guy won't be a goal machine due to his high physical what comes with dmr.Almost every skill point from gift or man of the match goes into his defense ,and also I never train defense on winger ,he always gets still% in defense . He will lose 20% on Sunday but will at the end of next season back on over 80% in defense due to stupid skills for his dmr
I have every season only 2 guys I give more greens,its my gk n one of 2 winger,all others stay average