The former Arsenal striker, who scored 30 goals last season, was out for six weeks with a thigh problem at the turn of the year and was ruled out for another lengthy period with a knee injury following the 3-0 win over Olympiacos in March.
"It's been a bit up and down for me this year,' said the 30-year-old, who has scored 18 goals this term despite his injury troubles.
"I've not been fully fit. That is where everything starts. You have to get that full season (of fitness).
"You can miss a couple of games, but not many. I missed half (the games) this season.
"I am gutted about that and I just want next year to stay fit, do well and compete again. I am sure we will next year."