Barcelona  striker Lionel Messi  says he is working hard to recover from injury after being ruled out for six to eight weeks with a hamstring problem.The Argentina international was substituted in the first half of Sunday’s 4-1 victory win over Betis and is not expected to play again this year.The injury is the third time that Messi has been sidelined this season and Barcelona have insisted he will not be rushed back. And Messi has now revealed his disappointment at being sidelined and has told fans he is working hard to get back to full fitness."As many of you know, over the next few weeks I will be working full-time on the recovery of this new injury, for me it is a pity not to play with my team right now," Messi wrote on his Facebook page."Now what is most important is to recover well to help my team-mates and give back all the support you have given to me in the best way I know how to do, playing football. Thank you for your messages!"Messi’s injury means he will miss Barcelona’s final Champions League games against Ajax and Celtic as well as Liga fixtures against Granada, Athletic Bilbao, Villarreal and Getafe.