The cure precedes the blow

איש יהודי במדרש יחידי.

"A Yehudi man" — in the Midrash, "unique (yechidi)."

The Megillah calls Mordechai "ish Yehudi," and the Midrash reads the word as pointing to his being singular and unique in his generation — a lone tzaddik who stood apart.

כי הקב"ה מקדים רפואה למכה.

For the Holy One, blessed be He, sends the cure before the blow.

Hashem prepares the remedy in advance of any plague — before a threat to Bnei Yisrael arises, He has already set the means of their salvation in place.

ומאחר שהי' המן רשע עריץ שרצה בכחו להשמיד כל היהודים.

And since Haman was a cruel rasha (wicked tyrant) who sought with all his might to destroy all the Yehudim.

Because Haman's power and his determination to annihilate the entire nation were so great, a correspondingly great counterforce had to already be present.

הי' מוכן בזה הדור איש יהודי וצדיק שהגין על כל הדור ובכחו קיים את כל בנ"י:

There was made ready in that generation a Yehudi man, a tzaddik, who shielded the whole generation, and by his power sustained all of Bnei Yisrael.

To match Haman's threat, Hashem had prepared Mordechai — a single tzaddik whose spiritual power protected and upheld the entire nation, the cure that preceded the blow.

Summary: Because Hashem always sends the cure before the blow, the very greatness of Haman's wicked power was matched in advance by the unique tzaddik Mordechai, whose merit shielded and sustained all of Bnei Yisrael in that generation.