Mattie has the operation to repair his groin on Tuesday and Peter says:
"It is a blow for us because he has done well but he has been carrying it for a few weeks.
"It has minimised him in training and meant sometimes he couldn't take part, which is not an ideal situation.
"Hopefully everything is going to be fine after the operation and it will rid him of it.
"A fully fit Matthew Etherington is very important to us and maybe it is a blessing he is getting it done early.
"He will hopefully come back fitter and stronger - and when he does he will be a big asset."
Malky Mackay is also on the injured list with a calf problem that prevents him teaming up with Scotland this week and Peter adds:
"Malky was never a guy to pick up injuries, he is normally one that plays 50 games in a season.
"It is unfortunate, but having said that Tomas Repka and Calum have been excellent in the last two games.
"I know he is desperate to make his name for West Ham United but now the challenge is there for him to get fit and get back in the team.
"That is the beauty of it - competition makes it better for us all."