[{"_id":{"$oid":"5f5f280ffa2236115655cb6a"},"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([{$set:{"Seasons":{$concatArrays:["$Seasons",["$Last_season_2","$Last_season"]]}}},{$project:{"Name":1,"avgGoals":{$divide:[{$reduce:{input:"$Seasons",initialValue:0,in:{$sum:["$$this.goals","$$value"]}}},{$size:"$Seasons"}]}}}])