[{"Name":"Rovilio Chipman","Last_season":{"year":"2010-2011","goals":10,"assists":1},"Last_season_2":{"year":"2011-2012","goals":1,"assists":12},"Seasons":[{"year":"2012-2013","goals":11,"assists":4},{"year":"2013-2014","goals":6,"assists":2},{"year":"2014-2015","goals":5,"assists":5}]}]

db.collection.aggregate([{"$set":{"AllSeasons":{$concatArrays:["$Seasons",["$Last_season"],["$Last_season_2"]]}}},{$set:{average:{$avg:["$AllSeasons.goals"]}}},{$unset:"AllSeasons"}])