Shawn Porter will indeed have to defend his IBF welterweight title against Kell Brook by July 19, as was known by both Porter and Paulie Malignaggi when they signed for their fight this past weekend, which Porter won impressively via fourth round TKO.
Porter (25-0-1, 14 KO) was granted an exception and allowed to face an optional challenger even though Brook (32-0, 22 KO) already had mandatory challenger status. All parties signed off on the agreement, so this is hardly Porter getting blindsided.
The IBF officially notified Porter and his team of the fight following his win:
"The IBF granted Golden Boy Promotions an exception to regulations for the Porter vs. Malignaggi fight. One of the conditions of the exception is that the winner must fight the leading available contender within 90 days. The leading available contender is No. 1 Kell Brook. ... Negotiations should commence immediately and be concluded by May 21, 2014. In the event that you are unable to come to an amiable agreement with Brook, the IBF will call for a purse bid."
This means that the sides have about a month to get the fight done, which isn't much time, but again, everyone knew this was going to come up. Golden Boy does have a pay-per-view on July 12, headlined by Canelo Alvarez vs Erislandy Lara, and given the fact that Porter took no punishment at all against Malignaggi, he could turn around that quickly and be back in the ring. One figures the Brook camp must be ready to go.
If Porter were to try to take another fight, he would be stripped of the IBF title. If Brook were to do the same, he'd have his status as mandatory challenger revoked, and Porter would then be ordered to face the next-best available contender -- or next-highest ranked, anyway.