Hazlewood was excluded from the World Cup so he could work on getting fully fit for the Ashes after an injury concern and the coach said the right-armer's style of bowling fit best with the game plan the tourists want to adopt as they chase a first test series victory in England since 2001.
"He (Hazlewood) has got an outstanding record, he's built up over the last few months, he missed out on the World Cup because we felt that he hadn't played much cricket," Langer said.
"We know he's an outstanding bowler, we know that the style of play we want to play against England, at his best he should execute those plans really well.
"He's bowled well the last couple of weeks and we hope he does a good job for us in this test match.
"He hits a great length, he's usually pretty miserly in his economy rate so that's what gave him the edge in this game.
"It's a hard decision. All these are really tough decisions. If it comes off we know what we're doing and if it doesn't we don't, but that's just the business we're in.
"It's a tough call though, a really tough call."
Pattinson was never going to be able to play the first three tests of the series, especially with matches two and three being held back-to-back.
The teams only get three days' rest between the Lord's test and the next clash in Leeds and given Pattinson has returned to the international arena for the first time in more than three years because of injury woes, it was decided to take a cautious approach.
"We collaborated on that one and I think it's been important for him in his return to cricket," Langer said when asked how Pattinson took the news he wouldn't be playing at the home of cricket.
"I think in the past he's felt … he had to play and had to play and had to push and had to push and in those instances he normally broke at some point, so (it was) really good collaboration between all of us.
"We just felt that having pulled up a little bit stiff after bowling (at training) that it was common sense to do everything possible to keep getting himself prepared and hungry and ready for the third test."
Pattinson made a successful return to domestic cricket last summer as he recovered from back surgery and bowled some hostile spells in the series opener.
Despite losing an entire day to rain at Lord's, Langer said Australia won't change its approach to the second test because he's expecting to make up time across the remaining four days as play will be extended each day.
But the weather forecast for England's capital city isn't looking great as more rain is expected for the rest of the test.
