Foster was injured during the Saints' 24-23 loss to the Detroit Lions on Sunday. The exact nature of the injury is unknown, though it was obviously significant enough to place him on IR.
Although the second-year pro had just one tackle and no sacks this year, he'll be missed as a rotational pass-rusher. Last year, Foster had three sacks as an undrafted rookie out of Illinois. He was averaging about 12 snaps per game this season, sometimes playing in a four-end pass-rush formation.
The Saints' entire defensive line has struggled to generate the same kind of pass rush as they did last year, so it hurts to lose any player that could have been a possible asset. They still have decent depth at defensive end, though, with starters Cameron Jordan and Akiem Hicks and backups Tyrunn Walker and end/tackle Brandon Deaderick. So it's unclear if they'll look to replace Foster with another end or look to add a player elsewhere on the roster.
As for the Crabtree/Davis move, it's hard to speculate on why the Saints wanted Crabtree back. He was presumably signed as insurance two weeks ago after starter Jimmy Graham suffered a shoulder injury, but he was released when it became clear that Graham could play (30 snaps in a limited role Sunday). So maybe he's still Graham insurance, or maybe the Saints liked enough of what they saw to potentially use him as an extra tight end and special teamer.
Davis was promoted from the practice squad on Saturday because the Saints were thin at linebacker with backups Ramon Humber and Kyle Knox both sidelined by ankle injuries. Perhaps the Saints are optimistic that one or both will be able to return. Davis will likely be re-signed to the practice squad if he clears waivers.