Poor Monastery Mentor. Now that it's restricted in Vintage and Sensei's Divining Top is banned in Legacy, the card barely sees any play. And it's much too powerful of a card for that to happen for too long. It deserves better.
Unsurprisingly, the card is at its best in older formats where there is a plethora of cheap spells to enable very explosive turns. In Standard, when you often had to cast the card as a 2/2 and hope to untap with it in order to make a Monk token, the card failed to make a major impact, mostly seeing spot play in Chained to the Rocks decks.
That leaves its prospects in Modern in an interesting place. There are numerous cheap spells around it but none of them are exactly free, which is when the card gets really degenerate. There's also a good amount of removal for the creature in the metagame, so you don't want to cast it until you have four mana, which can be a lot to ask. It's in a spot where it demands an answer immediately and takes over the game if left on the battlefield, yet can be answered without going to too much trouble.
The solution is to build a deck around it so as to get its good half as often as possible. Today's deck does that in multiple ways. First, it has lots of cheap cantrips to ensure the card is great if you untap with it. Second, you have lots of ways to protect it from removal in Thoughtseize, Inquisition of Kozilek, and Spell Pierce.
Last, and most importantly, the rest of the deck is built to be great against spot removal. There are very few other quality targets for your opponent to hit when you don't have a Monastery Mentor, so if they overload on answers for it, you can easily punish them by playing an attrition game with Lingering Souls and Snapcaster Mage.
Forcing your opponent to make awkward decisions on just how much removal they want to answer your main plan without losing to your backup plan is a great place to be since the answer isn't at all obvious and you have the card selection to play toward the half they are underprepared for. The cherry on top is that this deck does it in a proven shell of discard, cheap removal, cantrips, and good threats. If you're itching to break out your Monk tokens again, take a look at this one.