Abu Dhabi - Key Pakistan spinner Yasir Shah suffered a back spasm during team training on Monday, a day before the start of the first Test against England in Abu Dhabi.
The 29-year-old leg-spinner apparently hurt his back as he attempted to stop a bowler's back drive in the nets.
"Yasir's injury is concerning and could be a jolt," skipper Misbah-ul-Haq told a press conference. "We are looking into it and hope that he is okay by tomorrow morning."
Pakistan are relying heavily on Shah and fellow left-arm spinner Zulfiqar Babar to win the three-match series in the United Arab Emirates.
Shah took 24 wickets in Pakistan's 2-1 win in the Tests in Sri Lanka in July and was seen as the danger man for England, who were routed 3-0 by spinners Saeed Ajmal and Abdul Rehman here in 2012.
Shah was seen walking off the field in great discomfort, holding his back before getting treatment with the medical staff.
Misbah confirmed Shah hurt his back.
"He was walking well in the dressing room but we will have to take a final decision in the morning," said Misbah, hinting that one of the left-arm spinners who played against England in the first warm-up game last week could be summoned up.
Zafar Gohar impressed in the first warm-up game in Sharjah while another young leftie, Mohammad Asgher, has been impressive in Pakistan's domestic matches.
"Yes, we are looking into it and any of the two left-arm spinners could be called in will be fit in the morning," said Misbah.