Sorry to be a doubles idiot but Tsonga has withdrawn from the tournament. Is it possible someone like Gasquet could take Tsonga's place with Pouille? Or does Pouille just have to pull out now and let alternates get in? Thanks !
The answer from AvidTennisFan is wrong (it would only be true at Slams where there are no on-site spots).
At ATP tournaments, alternates will get this advanced entry spot, just like in singles, so in this case Pouille/Tsonga will be replaced by Mektic/Monroe.
Here is the rule from the ATP rulebook (p113):
2) If the withdrawal is from the advance acceptance list and prior to 12 Noon (EST) on the Friday before the event, then the next team on the alternate list is moved into the main draw.
3) If the withdrawal is from the advance acceptance list and occurs after the 12 Noon (EST) deadline then an additional on-site entry position is created for each withdrawal.
a) For both 2 and 3 above, the partner of the withdrawing player may enter doubles or sign in for the singles qualifying of the same or another event through the on-site entry method
So Pouille can still get in as one of the 6 on-site entries, but he has no advantage at all from being one of the originally accepted players.