Was it an illegal wiretap or was there a FISA warrant? If a FISA court issued the warrant, then it was not illegal.
Keep in mind that it is legal under US law to tap a foreign power *or their agent.* We know now that Manafort was at this time an undisclosed agent of a foreign power, same for Flynn given each of their undisclosed status as a paid to advocate for a foreign power. It would then have been legal to tap him.
In order for Manafort to be tapped, the FBI/DOJ probably provided evidence to the FISA court that he was an undisclosed foreign agent. So it would have been a legal court approved tap and probably very justified given what we know about the extent of the Russian efforts to help Trump and the Trump campaign’s knowledge of those efforts and eager acceptance of them.