Halak downplayed facing his former teammates all week, but was beaming after helping the St. Louis Blues beat the Canadiens 4-1.
"During the game, you just want to focus on hockey. You don't want to think about the friends on the other side or ex-teammates," Halak said after his 27-save effort.
After allowing a first-period goal to Jeff Halpern on the Canadiens' sixth shot, Halak was in lockdown mode the rest of the way, stopping the final 22 shots he would face.
"Obviously, when you play for a team and then you get traded, you want to beat them every time you play them," Halak said. "We play Montreal only once a year, so that was my chance this year. I needed to make sure I gave it my best.
Montreal was playing its first game without left winger Max Pacioretty, out indefinitely with a severe concussion and fractured fourth vertebrae as a result of a check Tuesday night at the hands of Boston defenseman Zdeno Chara. Pacioretty was released from the hospital Thursday.
"You're concerned about him, but at the end of the day, we're professionals," Canadiens captain Brian Gionta said. "We're going to go out there and put things behind us. I just think our effort wasn't there tonight."
"We know the situation with the trade," McDonald said. "For us, playing back-to-back, we needed a big effort. We're aware of the situation. We have injuries, younger players being called up ... it was a great overall effort."
The Blues' faint playoff hopes remained alive with their third win in a row as Halak, a playoff star for Montreal last season, got back at his former team in the process, outdueling Carey Price.
The Canadiens did all they could to downplay the long-anticipated matchup despite facing all sorts of questions about facing Halak and dealing with Pacioretty's injury.
"I think that's more of the outside world just making a big deal out of that," Canadiens center Scott Gomez said. "We knew we were going to face [Halak]. He played great. He did a good job."
McDonald gave the Blues a 2-1 lead 1:06 into the second period after D'Agostini poked the puck away from Benoit Pouliot in the offensive zone. McDonald found himself alone in front of Price and snapped a wrister past the blocker side.
Halpern opened the scoring for the Canadiens on Alexandre Picard's feed into the low slot, redirecting the puck past Halak 12:30 into the opening period. Backes tied it with his 24th of the season off McDonald's faceoff win with a quick shot from the slot, beating Price with 1:01 left in the first.
D'Agostini, who was also playing the Canadiens for the first time since being dealt to the Blues last March, sealed the game by converting McDonald's feed into the slot on a 2-on-1 with 1:29 remaining.
D'Agostini played a key role in downing his former team despite the main attraction being the duel between Halak and Price.
"That Halak-Price showdown, that's what everyone was talking about," D'Agostini said. "I had my opportunities in Montreal. I'm not going to say I didn't have my opportunities, but I just have found my groove hopefully here in St. Louis."
McDonald has 21 points in 19 games since returning from a concussion. ... Canadiens C Lars Eller, the Blues' first-round pick in 2007 and part of the trade that brought Halak to St. Louis, faced his former team for the first time as well. Eller has four goals in six games after scoring three in the first 57. ... The Blues will be without LW Alex Steen (high ankle sprain) and D Barret Jackman (broken index finger) for at least four weeks. Both were injured Wednesday night in an overtime victory in Columbus. ... Canadiens LW Andrei Kostitsyn has nine points in eight games.