Icardi is still Inter's best player.
Perisic league stats (Season Club start(sub) goals assist)
2016/17 Inter 31(5) 11/8
2015/16 Inter 26(8) 7/6
2014/15 Wolfsburg 21(3) 5/4
2013/14 Wolfsburg 24(9) 10/4
2012/13 Dort/Wolfs 12(13) 4/1
2011/12 Dortmund 8(20) 7/3
Perisc has scored 10+ goals only twice and has never had 10+ assists in a league season. Of course he can still be a great player to a club like Inter or Southampton, but in the context of United, he is as average as they come.
To give some comparison, Fellaini had 11 goals and 5 assists in 31 league starts in his last season with Everton (in a much tougher and faster PL) and he was only 25 back then. Who knows, Fellaini could have been a devastating scorer for Inter at 28.