During the call, Trump spoke highly of ousted prosecutor -- Viktor Shokin.
I am sure you are well aware that when Shokin first took office in 2015 as Ukraine's Prosecutor General (their version of Attorney General), he was praised by all for his pledge to wage an unflinching battle against corruption. However, as it soon came to light, Viktor was slowplaying investigations against former Yanukovych officials (the ones loyal to Putin), and in some cases, was outright hindering the investigations.
Pressure grew among the entire Western world for Ukraine to oust Viktor, who at that point had proven to be untrustworthy.
So no, President Zelensky did not need a heads up from Trump to ferret out bad guys in Ukraine. Trump wanted Zelensky to start an investigation (a commonplace Putin tactic) to give the appearance of impropriety per Biden.