The habitual anxiety in international fortnight that players might come back injured proved founded on this occasion, and Peter says:
"That is always the worry but it is a double-edged sword; one, that you want them picked for international duty because that shows the quality the player has, and two, there is always the possibility they get injured.
"It is disappointing but we have to get on with it; it is fortunate enough to get a player picked at that level but injury is the downside of it, for sure.
"Clubs that don't have any internationals can relax but part of football is that you do pick up these problems."
Peter insists that Calum, who has a hamstring problem, won't be rushed back to action, but on the other hand will be given every chance of returning when he has been checked by the club's medical staff.
"Strains are difficult ones because you can feel great one moment but there is a tenderness there when you are running," he explains.
"So you could feel better in two days or two weeks; some people recover from them quickly and some don't.
"With Calum it is the first problem he has had with us so it is a case of looking at his background, and we will give him to the last moment because we want everyone available.
"You want to give people as much time as possible - it is no use ruling him out now and then saying, come Saturday, he feels fantastic.
"So he will get treatment, we will assess it when he gets back, and if he doesn't make it, it will open the door for someone else."
Malky Mackay, who missed out on Scotland duty with a calf strain, could return to action against QPR, and Peter adds:
"Malky hardly feels it, but because it is in a bad position it is tricky; he has not trained with the team yet but calves, like hamstrings, are difficult to judge because they can feel great but still break up.
"We are not going to put anyone in jeopardy because we can't afford to take that chance - if they come back too soon they could miss out for a long time."
Andy Melville is another centre half to have missed representing his country this fortnight - he got as far as warming up for Wales prior to the England game but pulled out with a hamstring injury.
"When he went away I said to him 'you'd better make sure you do well!' but it is ironic that four centre halves have got injured recently, not to mention Christian being out," says Peter.
"It just shows you how quickly football changes - and sometimes people laugh at coaches for saying they take things day by day.
"It was Wales' biggest game for many years so it was unfortunate for him to miss out."
Anton Ferdinand, yet another centre half who had to miss international duty - he would have been with Calum Davenport and Nigel Reo-Coker in Azerbaijan this week had he not been doing rehab - could return at the weekend and Peter adds:
"It is more like a toothache with Anton, with a constant gnawing pain there; he has been out on the training pitch one day then rested, because with hamstrings, as I say, you have to have very careful."
Whether the injuries necessitate Alan Pardew looking to bring a loan signing in remain to be seen, but Peter says:
"You are always looking, as we would be if everyone was fit, because you are seeking to better the squad at all times.
"Right now, suspensions and injuries are kicking in and there are a lot of games coming up.
"We have been out keeping abreast of everything but hopefully it won't get to the stage where we have to rush anything.
"It is not as if we got people in to move others out; we did it to make the squad bigger because in the League Cup game against Southend, with some cup-tied, we had very few players.
"We want to do as well as we can in every competition and to do that you need a squad of players, but we were very thin on the ground.
"We got four boys in after the Coventry game and we looked a decent squad with challenges on each position - but now we have all these injuries in the same area."