"Very difficult first half, probably worst than last year," he said. "It is very difficult to keep the concentration and enough power, physically but especially mentally, when you can fight for a not very important position.
"We still have at least eight races. Last year the second half was very bad so we have to keep our concentration to make the maximum each weekend.
"I will decide [my future] in the next weeks, probably before Indy, because I cannot think too much during the race weekend because my brain is not free."
Newly re-signed team-mate Nicky Hayden finished sixth, then gave Rossi a lift back to the pits.