Newcastle are likely to hand the gloves to Rob Elliot for the visit of Norwich City on Sunday.
"It's a massive blow for us," McClaren said.
"Tim's a vital player, very important, and he'd get us 18 or 20 points a season. To lose that is devastating.
It just seems to be one disaster after another here but you have to get on with it. We have players with talent and ability -- once we get over this hump we will win games."
The manager, however, expressed confidence that Newcastle's directors would continue backing him as he tries to turn the season around.
The club are bottom of the league after eight matches without a win, and have conceded 17 goals even with Krul in the team.
"I have the belief I have their (the board's) loyalty and as long as we continue making progress, which I believe we are and they believe we are, then we keep going," McClaren said.
"You don't have loyalty if people think: 'This ain't going to get any better.'
"We're planning together. We realise that the next transfer window is going to be very important for us -- it's going to be huge.
"We've talked a lot. They absolutely realise this is a tough job. There's no feeling that everything in the garden is rosy. Even if we win a few games, there needs to be changes.
"This is Newcastle, so there's always scrutiny on you. Always. From day one -- but I'm experienced enough to know how to handle it and it's not really a problem. The problem is bloody winning football matches," he added.
(Reporting by Simon Jennings in Bengaluru; Editing by John O'Brien)
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