
Harry Maguire and Lisandro Martinez were unavailable due to suspensions in the capital, while Leny Yoro was ruled out of the game with an injury at the eleventh hour. Michael Carrick said he learned United would play with a makeshift backline against Chelsea "throughout the day" on Friday.
Heaven made his first start for Carrick and Mazraoui played as a centre-back in a United back four for the first time, but the pair excelled at Stamford Bridge, which led United's interim boss to say: "They were fantastic."
Speaking to the Manchester Evening News, Lammens said of the pair: "I mean, in general, it's not always the easiest. I think at the start of the season, we also had it a couple times where players came in and came out, especially at centre-back.
"It's a position where you need a little bit of confidence, where you need a little bit of, like, getting used to it. So it's difficult when, you know, players are out of the team for sometimes a couple of months, or a couple of games, and you have to come in.
"I mean, of course, I have the biggest, biggest trust in them because you see it every day in training, and it's also the biggest change for me as well. If you see in this club, everyone in the squad is able to play and able to perform at that level.
"So you know, when their moment is there, they have to be ready for it. They did it today. They did a very good job. I think it didn't really feel like there was a lot of change, to be honest. So I can't praise them enough as well."
Asked what they did well, he said: "I think just, like, not trying anything too difficult, sometimes you come in and you maybe want to show yourself too much, especially for young guys, it's not easy.
"I think they both did a good job, and, of course, Nous, playing centre-back when he's normally a right-back, which is not always the easiest, but I think today, we couldn't tell he wasn't a centre-back his whole life, so... playing together as well, helping each other out, and also the team helping them out.
"It was a hard-fought team win. Everyone performed, especially those two guys [Mazraoui and Heaven], who I have to praise a lot."
United's win at Stamford Bridge put 10 points between themselves and Chelsea, but it was also a big win for Carrick after suffering a 2-1 defeat to Leeds at the beginning of the week.
"He's very demanding, of course, but he also knows that in the Premier League, in the way we are, you can win and lose some games," said Lammens about Carrick's response to the Leeds setback.
"It's always the most important thing is how you react to it. I think last week against Leeds, we were not good enough, especially at home, but we also knew that today was a very good chance for us to take those points and take a very good step into the Champions League.
"It's kind of a very good, important opportunity for us to take it and to like turn it around a little bit, but we also know there's no, no single threats or like no, how you say it… Yeah, we also know we believe in ourselves.
"And sometimes it’s difficult to think about the first couple of games when we had the winning streak, and that feeling when that comes back, and we want that feeling back. So that's also what he was speaking to us about to get that feeling back and to start a run. And I think today was a very special one."
Lammens revelled in the full-time celebrations with the away section. "We've had it a couple times this season, away against very good teams as well. Arsenal, here, other places as well, you know," he said.
"At home it's special, but it’s also always very special away, and you can feel the support of the supporters, especially in the first half when you're playing in front of them, but also in the end, just the feeling with them and the way we can celebrate with them as well, coming here, and I mean, we had to fight for it. It wasn't the prettiest, but that sometimes the most beautiful thing."
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