Absolutely! Typical "God substitutes" include things like people we see as heroes or idols. Such heroes or idols can be individuals or groups. We can also use things like science or philosophy to replace God with. I remember a man I know who was seeking a "higher power" which turned out to be psychology.
So because I admire Carl Sagan, he's a substitute God to me?
Only if the meaning of "God" becomes stretched to meaningless, I think.
Do you worship Carl Sagan? Do you believe that he can help you in your daily life (despite being dead)? Do you think that his writings are perfect and cannot contain any mistakes or inaccuracies?
I rather doubt it.
Sagan was entertaining, well informed, and very good at helping others to become better informed about reality. But he wasn't a God in any sense, and he doubtless made some mistakes - because everyone makes mistakes* - and so it would be the height of foolishness to accept his word for anything, without checking against the reality he purports to describe.
Nullius in Verba, as they say at the Royal Society.
* Even me. I made a mistake once; I thought I was wrong about something, but it turned out that I wasn't.