These are the numbers for all the seasons Lukaku was playing in the Premier League.
2012/13 - 17 league goals
2013/14 - 15 league goals
2014/15 - 10 league goals
2015/16 - 18 league goals
2016/17 - 25 league goals
2017/18 - 16 league goals
2018/19 - 12 league goals
In the first season, he played for a West Brom side that finished 8th (their best ever). From 2013 to 2017, he played for an Everton side that flitted between pushing for Europe and mid-table. In his last season at United, he wasn't an automatic starter.
Even if you take out his "one very good season", he averaged 15 league goals every season. That is very impressive considering the sides he played in.