Monahan will be receiving a league minimum contract loaded with performance bonuses, if of course he is even offered a contract.

I get that but there's no way for us to do that kind of contract on this site

I'd have to double check how everything works, but I think a performance bonus contract would hurt his trade value. Somebody would have to carry that into next year if they don't have the cap space available this season to cover it.
Much simpler to just take the risk and sign him to a standard contract that we can retain on to control his cap hit.

Not necessarily would it hurt his trade value.

He's injury prone so it's an insurance policy. He plays X# of games, gets bonus 1. Scores X# of goals, bonus 2. Etc (I'm aware you know how it would work, just laying it out)

If he's having a stellar year, MTL ends up paying the bonuses and then trading him. This saves the receiving team money.

If he's having a less than stellar year there is the possibility he doesn't reach his bonus points and is a cheap vet. Big market.

If he has another injury prone season and is on the LTIR at the deadline, the receiving team only has to show they have space to have him on the roster prior to him being sent back to LTIR for a potential late season/playoff return. In this scenario the receiving team would then only have to maintain space for a league min contract and opens up to more teams/cap strapped teams.

If I were to go the standard contract route I wouldn't want to give him more than 2 mil for the year. Since it's likely he's injured more often than not.

The problem is that he might hit those bonuses after being traded, putting the acquiring team in tight cap situation.
Say he has a bonus for 60 games played and plays his 60th game after being traded. I "think" the new team is on the hook for that bonus.
Assuming they are already at the cap for the current season, that amount would come off their cap for the following season.
You can structure bonuses differently in an attempt to avoid that of course, but a Monahan trade can be complicated enough without extra kinks. Montreal can afford to forego bonuses and should, in order to maximize his trade value.

(I'm not 100% sure bonuses aren't pro-rated, I'm just cautioning that it's a possibility.)