Alonso has yet to finish on the podium since rejoining McLaren in 2015 and his current contract expires this year.
He said Indy showed him he can win in series other than F1 if he ever decides to leave. "If we are not winning before September, when I will make a decision, I will not stay".
McLaren executive director Zak Brown said on Wednesday that he had "serious concerns" about Honda, which he said appeared "lost".
"I think it will be purely dependent upon does he think we're going to have a competitive race vehicle next year", said Brown.
"He wants to win, he wants to put McLaren again as a contender position for the championship".
McLaren boss Zak Brown revealed yesterday that McLaren is fast approaching "a fork in the road", when hard decisions will have to be made.
"In F1, a third World Championship is my biggest priority and a Formula 1 auto is the best thing I can drive after 16 years' experience". But the team have been uncompetitive for all three years, largely because of a lack of performance from the Honda engine, and that is causing tension within the team. "I started when the calendar was 16 races, plus the tests, and now we keep increasing the races year after year", said Alonso.
"After three years we are not in that position, so things have to change, I guess for the team".
"That's the only thing I can say now until I sit down with myself in September or October - after the summer, as I always said. Until I sit down with myself, in September or October, whenever, I can not say anything 100 percent".
Alonso missed the Monaco GP to contest IndyCar's Indianapolis 500, but said that experience had "not really" influenced his thinking about his F1 future.
"A third world championship is my biggest priority", he said.
However, with new owners Liberty Media keen to evolve its investment by bolstering F1's reach in the United States and Asia without turning its back on traditional European races or events in Russia, Azerbaijan and the Middle East which pay higher than average hosting fees, adding to the schedule has been mooted instead. "You never know", he smiled, "you can not be 100 per cent certain in June about a decision for next year that I don't even start to consider".
"Now they keep increasing the races year after year and we are at a number that is quite demanding".
The 2017 calendar has 20 races but next year will see it grow to 21 with France and Germany returning and Malaysia leaving. "If the calendar stays between 20 and 21 races, I would be happy to continue but it if increases it is not for me".