On-loan Middlesbrough defender Gibson had been playing through the pain barrier for a number of matches and was forced to sit out Saturday's 2-2 League One draw with Portsmouth.
Moore told Tranmere Player: "We're waiting - he's going to see a specialist on Wednesday. Nobody seems to know what it is so we've just got to adhere to what Middlesbrough want.
"Until we know what it is then basically the boy can't play. He's a big miss, we've got him and (James) Wallace out at the wrong time."
Skipper Wallace has damaged medial knee ligaments and the former Everton midfielder joins forward Jean-Louis Akpa Akpro and winger Abdulai Bell-Baggie on the sidelines.
"We've had to cope with the other two being missing for a while and these two have come at the wrong time," Moore added. "There is no right time but we've got some tough games coming up."
