Jake Bidwell is set to face a long injury lay-off after suffering a dislocated shoulder against Burton Albion.
The summer signing from Brentford, who missed just one game last season, was stretchered off in the closing stages of the 1-1 draw at the Pirelli Stadium.
And he is set to be absent for two to four months and will undergo a scan on Wednesday to determine the extent of the absence.
Hasselbaink told the club's official website: “Jake will have a scan tonight, but it doesn’t look good.
“It looks like he's dislocated his shoulder and we’re going to be without him for a while, which is very disappointing.”
It means Bidwell will miss Saturday's west London derby with Fulham and will also miss a reunion with high-flying Brentford.
Hasselbaink felt the draw at Burton was two points dropped, rather than a point gained as Sebastian Polter cancelled out Jamie Ward's opener.
“I think it's two points dropped. I felt we deserved all three but it wasn't to be tonight. The reaction to going a goal down was positive. That shows character.
“What frustrates me is that we are having to come from behind – and that needs to improve.”
He added: “It was a bitty game really. I was pleased with how we pressed the game early on. The tempo was good, but it can be better.
“We battled well and created chances in the first half that we should have taken.
“I felt we were in control at 0-0 at half time. They made it difficult at times but we should have made more of our quality in the final third.
“Their goal came at a time when we were on top, but the game swung on a massive decision 60 seconds before.”
