Ben left the pitch at Vicarage Road after just 35 minutes, adding to the woes already suffered in the central defensive area.
"We've got four centre-halves out all in all," lamented the boss. "We've got Anthony Gardner, Deano and Ledley - and now another one.
"Ben has pulled a groin. He slipped in the first half and has slightly done a groin - we'll have to see how that is in the next couple of days.
"With the three other lads out it's going to be a bit difficult for us in the next couple of games."
"What hurts you is we've got four players out now in one area and it hurts your building, it hurts your foundation.
"Ledley hasn't done much yet and is unlikely to start the season, Anthony has got a viral infection of the chest which is going to take some time and Deano has been concussed and we have to wait for the all-clear from the specialist.
"Now Ben's done a groin. You have to have these matches, but to keep picking these injuries up all in one position - that's what is hurting us."
Glenn admits the injury sustained by Ben Thatcher at Watford on Saturday gives him a defensive headache as the new season approaches.