Howley has already seen his squad hit by a number of serious injuries, with Bradley Davies, Ian Evans, Luke Charteris and Alun Wyn Jones all on the sidelines at the moment.
Jones' coach at the Ospreys Steve Tandy said: "He dislocated his thumb and I think the bone may have popped out as well.
"We'll review that on Monday but it doesn't look good for him though."
Howley was not the only Six Nations coach to lose players this weekend, with France youngster Gael Fickou set to miss Les Bleus' training camp after suffering an ankle injury in Toulouse's win over Treviso.
The 18-year-old came off the bench for the four-time European champions but had to be replaced as a result of the knock, and now looks unlikely to make his debut for France having been named in the squad last week.
Toulouse head coach Guy Noves said: "Fickou has an ankle sprain which will keep him out of France's training camp."