On-loan Manchester United midfielder Collyer, 21, limped off inside 10 minutes of the second half of his full home debut in Saturday’s 2-1 win over Preston at The Hawthorns.
Boss Ryan Mason said the midfielder felt discomfort in his hamstring and scans have subsequently revealed damage.
Mason also revealed Collyer has suffered issues with the muscle previously at parent club Manchester United.
The head coach gave no timeframe for a return, but Albion face an extremely quick turnaround from the trip to Vicarage Road and face another testing away trip to Ipswich’s Portman Road for an early 12.30pm kick-off on Saturday.
Mason said: "Hopefully, it's not too bad. He had a few injuries at Man Utd last season in a similar area.
"He's a young boy, he's still growing, he's still developing, and we need to be aware of that. So, yes, the scan come back, it's something that's going to keep him out, but hopefully not for too long.
"We have the scan where we can work off but it's really important to treat the body, treat the symptoms and how the player feels. I don't like putting a timeframe on injuries because then that limits the mental side and the body's capacity to get back sooner."
Albion remain without captain Jed Wallace for the clash at Watford, who are under returning boss Javi Gracia at home for the first time.
Wallace missed the Preston clash with a calf injury and is not expected to return in time to face Ipswich on Saturday.
Isaac Price, one of the goalscorers in Saturday’s 2-1 win, is fine after limping off with a dead leg late on.