Rav Zilbershtein actually said that he himself is not worthy to call upon the Jewish people to daven for President Trump, as when it is the greatest person of the nations, the greatest person of the Jewish nation should be the one to call upon the people to daven.  

That being said, he did say that in his opinion everyone is obligated to daven for his speedy recovery out of gratitude for what he has done for the Jewish people of Israel. Rav Zilbershtein added that this is how Rav Elyashiv paskened previously.

1. it is surely debatable whether Trump is the greatest man in the world. He might be the leader of the strongest nation, but that is also debatable (maybe China, maybe Russia, maybe another nation).

2. when he defers and says greatest of the Jewish people should make the call does he mean the gadol hador or the leading statesman or the greatest Jew alive? would it be Rav Chaim Kanievsky? Benjamin Netanyahu? someone else?

3. Refuah Shleima