Despite opening the batting on what was seen as a favourable pitch for the batsman, England were bowled out for just 192. Ajmal was the pick of the Pakistani bowlers, claiming figures of 7-55.
Prior, who was the pick of England's batsman on 70 not out, believes that the tourists must improve their shot selection against the spinner.
"It's been a tough day. We haven't performed as we'd have wanted to," Prior told Sky Sports News. "It's obviously a pretty good batting surface. A mixture of some poor shots and a bit of luck was to blame.
"Saeed Ajmal has got the variations. Any spin bowlers that spins it both ways is tricky. We've got to find a way of dealing with him a lot better than we did today."
Pakistan will begin day two 150 runs by England.