#3 Romelu Lukaku
Selling Romelu Lukaku is definitely something Chelsea regret. After two fantastic loan spells at West Brom and Everton where Lukaku scored 17 and 15 goals respectively, many thought it was finally time for him to stake his claim as Chelsea’s number one striker. But to fix the lack of quality up front from the previous season, Chelsea instead brought in Diego Costa and Lukaku's loan deal at Everton was made permanent, clearly not learning from the mistake they made with Sturridge.
Three more successful seasons later and Chelsea were ready to bring the Belgian back after Costa fell out with manager Antonio Conte, but lost out to Manchester United. Just like Sturridge, Chelsea could have used someone like Lukaku when their options up front were lacking quality.
Since his first loan spell at West Brom, Lukaku has scored 93 league goals. A really embarrassing situation for a club who told the Belgian just 3 years ago that he wasn't good enough.