Since scoring a podium in the first grand prix of his Yamaha return at Qatar, Rossi has come home inside the top five just once, at Jerez.
He has also crashed twice in the past two races. At Le Mans he was able to recover to 12th, but at Mugello he suffered an early exit after making contact with Alvaro Bautista heading into Turn 3, Poggia Secco, on the opening lap.
Team-mate Jorge Lorenzo meanwhile took a dominant win and with it the championship lead, and his total is now more than double Rossi's.
"The Barcelona race will be very important, especially after recent results," Rossi said. "I want to get on the podium in Spain.
"For a few days after I had the crash at Mugello, I hurt everywhere.
"Fortunately I was able to start training immediately and I did not waste any time.
"It will be very important to make a good qualifying [in Spain] and start from the front."
Yamaha team director Massimo Meregalli backed Rossi to arrest his recent slide, adding: "Valentino needs a good result here as he has had some bad luck in the last two races.
"We are ready for him to recover what he has lost over the last two rounds and perform as we know he can in Catalunya."