Dutchman Engelaar suffered a fractured leg in his first game of pre-season for Heart and although he will not undergo surgery, the former Netherlands international is not expected to return to action until 2014.
"He's not having an operation, he's letting it heal naturally. In three months' time he should be ready to start to run," Aloisi said on Wednesday.
"And then we'll see how quick it heals from there. We're still not sure on the exact time he'll be out for, if it ends up being four, five, six months. But we're confident he'll be back for the (remainder of the) season."
The search is now on for a player capable of acting as a temporary replacement for Engelaar, but Aloisi will be well aware that players of the calibre of the former PSV Eindhoven and Schalke man are few and far between.
"We're working through the solutions now," he said.
"He won't be out for the whole season, we're confident he'll back. We have to work through hopefully getting a player in - an injury replacement - and seeing what we can come up with.
"We're confident that we'll still put a strong squad together, even (with) the unfortunate injury to Orlando.
"You don't want to wish it upon anyone, let alone your marquee player. They're things that happen in football, you accept it, you move on and you come up with solutions and that's what we ended up doing.
"A day later we started speaking about what (we can) come up with now. And then we're working through that with (general manager) John Didulica and the club."
In another set-back for Heart, Engelaar's compatriot Rob Wielaert also picked up a training knock which is likely to keep the veteran centre-back out for the rest of pre-season.
"Rob Wielaert, he rolled his ankle the other day," Aloisi said.
"We're not sure how long he'll be out for. We're still hoping and believe he will be right for the first game of the season."
Finally, Aloisi endorsed the ambitious targets set by newly appointed captain Harry Kewell, who said the club will be aiming to win the title in his first season as skipper.
"I think that everyone believes they've got a chance of winning it and we're no different," the Heart coach said.
"We're putting a squad together to be competitive. You have to aim high and that's what we're doing. And I'm sure that every other club in the A-League's doing the same."
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