The Reds lost for the eighth time in 10 games. Wade Miley (2-2) gave up two runs on five hits in six innings.
Arenado hit an RBI single in the first.
The Cardinals added a run in the sixth when Knizner doubled off the left-field wall.
Reds CF Tyler Naquin turned 30 on Saturday.
Reds: INF Mike Moustakas, who hasn’t played since April 14, continues to recover from a non-COVID-19-related illness that has kept him on the 10-day injured list retroactive on April 20. “I’ve had a viral infection,” Moustakas said. “I don’t want to get into too much detail. It was bad and I wasn’t feeling good at all. I wasn’t able to fight through it so I got put on the IL. Last couple of days, I’ve been feeling a lot better. The cure for me was just time.”
Moustakas, who is hitting .289, said he could return next week. He will be eligible to return from the IL on April 27.
Cardinals: Molina exited after batting in the fifth inning Friday because of soreness in his right foot. Molina had raised his batting average to .339 after he homered and doubled. “I talked to him. He may be the toughest man alive and he said ‘I can play.’ I’m not too worried about it,” manager Mike Shildt said before the game.
Reds: RHP Luis Castillo (1-1, 6.05) will make his team-high fifth start of the season. He lost to St. Louis in season opener in Cincinnati. He is six strikeouts shy of 600 for his career.
Cardinals: RHP Jack Flaherty (3-0, 3.80) had a no-decision in his opening day start at Cincinnati. The Cardinals have won in each of his first four starts this season, averaging 11 runs per game.
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