Not happening for multiple reasons.
The MLS schedule is different, it concludes around December and starts up again around March/April.
The most important thing is that Saudi won't let Cristiano miss this opportunity.
He's been there for 3 seasons and has been awarded around 40 penalties so if they feel there is any chance that Messi can reach it first the refs might become less strict and allow him to reach this milestone as it serves as a promotional boost for their league.
MLS is switching to European style structure from next season.
So Messi can play minimum of 21 matches and maximum of 28 before next calendar begins in Summer
On an average he scores 20/21 goals in 24/25 matches
He needs 70 goals so I think 80 matches
If he plays all possible 28 matches and 35 odd games in the next calendar then he can get to 1000.
So there's huge possibility that Messi if stays fit can reach 1000 goals by the end of next season