The Republic of Ireland international is struggling with a recurrence of the calf injury that troubled him during the second half of last term.
Lawrence featured twice for his country as they won the Carling Nations Cup this summer, but the 29-year-old was forced to sit out Monday's friendly against Havant & Waterlooville and saw a specialist on Tuesday.
Cotterill felt the busy international schedule was part of the problem and told the Portsmouth News: "Liam has played a lot football, he's not had a break in the summer.
"He rolled into internationals so has probably had only four weeks off, which is probably not enough time.
"We just need to make sure he is okay, otherwise we can't play him because it would set him back weeks."