The Senators trailed by one goal heading into the third period on Saturday and couldn't spark a comeback as the Blues dominated the final 20 minutes to hang on to a 3-2 victory.
"They're making hockey plays and we're not," said MacArthur. "We're not making good passes. They're going tape-to-tape and moving it and we weren't, so it's tough to get momentum. It's just got to be crisper."
Alex Steen's second period goal held up as the winner.
Maxim Lapierre and Vladimir Tarasenko also scored for the Blues, who improved to 13-6-1, while Brian Elliott stopped 24 shots.
"It was good for the boys to get the win," said T.J. Oshie. "It was good for us to get some goals for our goaltenders as they've been working hard and playing outstanding all year and we haven't been getting them as many goals as of late so it was nice to get (Elliott) the win."
Ottawa got goals from Mika Zibanejad and Erik Karlsson as Robin Lehner, making his second consecutive start, stopped 30 shots.
Lehner could hardly be faulted for his effort, although the 23-year-old felt he could have been better.
"First goal is tough when it comes right off the bat and he opened me up, it's something I've got to work on," said Lehner. "I think the tip on the third goal too. It's just a reflex to start reaching with your stick to deflect it into the corner and out of the rink and he tipped it back. That stuff bugs me and I've got to work on it, but I felt like I came back good in the third and had some good saves."
The two teams meet again Tuesday in St. Louis for the second and final time this season.
Ottawa (9-6-4) made a habit of playing poorly in afternoon games last season, going a dreadful 3-7-2 in games starting before 5 p.m. Eastern— and it seems to have picked up where it left off.
When asked about the Senators record in matinee meetings Karlsson made light of the team's struggles.
"It's just another stat to keep track of," said Karlsson. "We haven't won a lot of afternoon games, we're all aware of it, but it's not something we really think about. We can try to do different things, but we try everything we can to win games, but they're a good team and we're going to lose sometimes."
Trailing 3-2, the Senators started the third period on a power play, but were unable to capitalize on the opportunity.
St. Louis outshot the Senators 15-5, dominated the face-off circle and won numerous puck battles in the final 20 minutes.
"They outplayed us in the third period, which was the only thing that would be disappointing in the game," said Senators coach Paul MacLean. "I thought we were competitive and played hard against a hard team."
The two teams exchanged goals in the second, but it was Lehner and Elliott who stood out.
Elliott made a huge glove save on Colin Greening early in the period and minutes later Lehner bailed out Karlsson on a brutal giveaway that led to a Tarasenko point-blank chance.
It was the second time in the game that Lehner had to bail out his captain after a giveaway.
St. Louis made it a two-goal game at the 12-minute mark as Steen tipped Jay Bouwmeester's shot on the way to the net to beat Lehner.
The Senators managed to cut the lead to one on Karlsson's power-play goal from the point late in the period.
"I didn't see it coming through all the bodies," said Elliott, who spent part of four seasons playing for the Senators. "Those are tough ones to swallow, especially when you're up 3-1. That goal makes it a tight game. We battled back and kept things to the outside for most of the rest of the game. Obviously when you're not going to the box, that helps as well."
Both teams had a solid first 20 minutes, but it was the Blues who went into the break with a 2-1 lead.
The Blues opened the scoring on their very first shot of the game as Lapierre got around Cody Ceci and went on to beat Lehner through the legs.
"Every once in a while I try to create some offense and things like that," said Lapierre. "It worked tonight, but it doesn't work every night."
Ottawa tied the game on the power play at the 16-minute mark as Bobby Ryan came from behind the net and found Zibanejad in front. But a mere 30 seconds later, St. Louis regained the lead as Jori Lehtera found Tarasenko all alone. Tarasenko's goal was his team-leading 12th of the season.
The Senators now head out for a five-game, eight-day road trip starting Monday against Detroit. The Blues head to Winnipeg to take on the Jets on Sunday.
Notes: The Senators' Chris Phillips was a late scratch. MacLean said Phillips was experiencing discomfort from an issue that bothered him earlier in the week. Marc Methot remains sidelined (back, indefinite). Erik Condra and Curtis Lazar were healthy scratches for Ottawa. The Blues' Joakim Lindstrom, Chris Porter and Chris Butler were healthy scratches.