Premiership leaders Bath are also likely to be without winger Joe Cokanasiga when they return to league action this weekend after an eight-week break.
Cokanasiga went off injured last weekend as Bath ended a 17-year wait for a trophy by clinching the Premiership Rugby Cup against Exeter.
“Joe took a bang to the lower leg,” Van Graan said.
“[I’m] not going to speculate on length [of absence], he’s under the careful watch of Rory Murray [Bath head of medical] but hopefully it’s not too serious and we see him back soon.”
Bath are six points clear at the top of the table and take on West Country rivals Gloucester on Sunday at the Recreation Ground in a weekend of derby fixtures.
After being beaten in the final by Northampton Saints last season, Bath are in prime position to go one step further and win their first Premiership title since 1996.
However, Van Graan said the team are focusing only on their next match.
“We don’t pay too much attention to the outside noise – we know there’s a lot of speculation and people have got expectation and that’s brilliant because people care about the club and they care about our team,” he said.
“For us it’s business as usual, [we] started at zero two days ago – control the bits we can, don’t look beyond Sunday three o’clock.”
