Humphreys has sustained a fresh injury in addition to the sore Achilles he suffered on the club's tour of Holland, while Clark is struggling with a hernia problem which could require surgery.
Director of sport Chris Turner told the club's official website: "Ritchie has had an Achilles problem rolling over from last season.
"He had some injections in it to settle it down but in the first period of the game last week he felt something go and the scan has revealed a new injury on top of the old injury.
"Ben thought earlier in pre season he was getting a groin strain but it seems that unfortunately it's a hernia. You can play on with a hernia but it gives you discomfort and it plays on your mind so you're better off getting it done straight away and then getting back to fitness as soon as possible."