Former Chelsea boss Avram Grant opined that Diego Costa's sale to Atletico Madrid has hurt the Chelsea and believes the club should have put up more of a fight before letting go of the striker. Grant has slammed Conte for selling Costa and added that his departure goes some way to explaining Chelsea’s struggles this season. Costa scored a total of 22 goals in 42 games last season to help Chelsea win the Premier League and reach the FA Cup final. Costa and Costa fell out during the campaign after the Spanish striker declared his intentions to move over to the Chinese Super League. Conte's reign at Chelsea began to suffer a set back after he told Costa via a text message that he was surplus to requirements. Costa felt disrespected by Conte behavior and he embarked on a self-enforced exile in Brazil before the Brazil born Spain international was eventually allowed to rejoin former club Atletico Madrid in January in a £57m move while Chelsea signed Alvaro Morata from Real Madrid. Grant Chelsea should've fought harder to keep the Spain striker. "One should not have let Diego Costa 's departure be so simple and thought that everything will resolve," he told Sky Sports. "Top teams today release no one. There are new times and there is a lot of money in the sport. It affects you. If you pick out Diego Costa you can see one of the reasons that Chelsea has been weak this year." The former Black Stars manager believes the Blues could struggle to attract the world's best players if they fail to qualify for Champions League football next season, but added that the 2016/17 season Champions will definitely bounce back from the disappointment of the current campaign. "Of course it will affect them because the top players want to play in the Champions League but it is not the first time. Chelsea was not there two years ago and even in the year that they won the Champions League they finished sixth in the Premier League. Avram Grant believes Chelsea have missed the Spaniard's presence up front this season "It is not the first time, of course, it will affect them but the most important thing is how they react. Until now, after they have had a bad season, Chelsea have reacted very well." Grant moved to England in 2006 to become Technical Director of Portsmouth before being appointed Director of football at Chelsea in July 2007. But was later appointed the clubs manager following the departure of José Mourinho. The Israeli won 68.8 per cent of his Premier League matches in charge of Chelsea and took the club to their first ever Champions League final and second place in the league that same year. Grant believes Conte should remain at Stamford Bridge for the final year of his contract despite the club's struggles this season and despite being strongly linked with a move at the end of the season. Grant added: "Had I been in his shoes, I would really like to stay a year to show everyone that I'm keeping my weight."