Fleming has been out of action since the middle of last month after sustaining an abdominal tear during the triangular one-day series in India which also involved Australia.
"I was hopeful it would improve with rest and I would not need an injection but that hasn't been the case, which is a worry,"
"The tear is very deep and I'm in a race against time not only for the Test but just to get some cricket under my belt."
Pakistan beat New Zealand, who are being captained by Chris Cairns, by 124 runs in Lahore on Monday to take a 2-0 lead in the one-day series between the two sides.
The next match in the five-game series is in Faisalabad on Wednesday.