Michael Johnson and Craig Gardner have been forced to pull out of the England Under 21 squad.
Manchester City midfielder Johnson is struggling with a troublesome groin injury, while Aston Villa ace Gardner is also unavailable because of injury.
Queens Park Rangers forward Dexter Blackstock has also withdrawn, as has on-loan Morecambe goalkeeper Joe Lewis.
England boss Stuart Pearce has replaced Lewis with Manchester United youngster Tom Heaton.
The quartet will miss the Euro qualifiers with Montenegro on 12th October and the trip to Cork to face their Republic of Ireland counterparts on 16th October.
Pearce insists his side will be looking to take six points from the two games.
No slip-ups"We feel that we have picked up two very good results away from home, but we're not top of the group," said Pearce.
"It tells us straightaway how difficult this group is, and we can't afford any slip-ups.
"Montenegro certainly won't be a game we are taking lightly; we still have a job to do against them, although we won away.
"We have to make sure we beat them again at Leicester, because there are three very important points on the table."