Jonah Williams injury happened before training camp, so he was placed on the PUP (Physically Unable to Perform) list. After the start of training camp, if you get injured and placed on IR, then you have to stay on IR all year long. If you make the initial 53 man roster and are injured, after the the rosters are "final", then you can be placed IR with the ability to be recalled after 6 weeks. Once a player is recalled, they have a 2 week roster exemption, where they can practice but not play until 8 weeks. The team then has 21 days to make a decision. if after 21 days they are still injured, then the player can go back on IR for the rest of the year. The PUP list rules are outside of the IR rules. PUP rules allow players to come back to the roster after six weeks. Once able to practice, the player from the PUP list has to be either re-instated to the roster within 6 more weeks, released, or they can be placed on IR for the rest of the season. The six weeks does not start until the player is able to practice. In Jonah's case, his six weeks has not started yet, as he has not yet practiced with the team. This is how they can be kept on the PUP List like AJ McCarron was in 2014, if they are unable to practice. Players on the PUP or IR list do not count against the 53-man roster while they are out, but they do count against the 90-man roster limit.
With Jonah Williams, once he is cleared to practice, he then would have 6 weeks to be re-instated or placed on IR, meaning that he could return all the way through December.
See below:
https://www.cincyjungle.com/2018/8/31/17797904/explaining-pup-nfi-ir-other-injury-nfl-roster-designations-cuts-53-man-roster