Niemann allowed three hits and struck out a career-high 11 in 6 2/3 innings in a 8-0 victory over the Seattle Mariners on Friday night.
He set the club record for any month for any Rays pitcher with a 1.06 ERA and a 3-0 record.
"Every since I came back off the DL, I've wanted to get back and contribute to this team," Niemann said. "I didn't get off to a great start."
He began the month with a 2-4 record and a 5.00 earned run average. Before he went on the DL, Niemann was 1-4 with a 5.74 ERA.
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Since returning, he is 4-0 with a 1.88 ERA, lowering his overall ERA to 3.51. Over his last three starts he has a 0.87 ERA.
"He's been outstanding the whole month," Rays manager Joe Maddon said. "It's great to see him get back in this manner."
The Rays hitters made it easier for him with an eight-run second inning.
Ben Zobrist hit his 14th home run, a two-run shot. Over the past nine games, Zobrist has 15 hits in 37 at-bats (.405).
"We got up early so it was nice to have like a no-stress game there," Zobrist said. "I feel more like me as a hitter. I felt like early this season and a lot last season I did not get to the point where I felt as comfortable like I have been the past couple months."
Maddon said Zobrist "quietly is having a really, really good year."
He's hitting .279 with a team-leading 56 RBI.
Casey Kotchman had three hits, scored twice and drove in a run. In the second inning alone, he had a single and a double, scored twice and drove in a run.
Kotchman, who hit .217 for the Mariners last season, is now hitting .329, third-best in the American League.
Mariners starter Erik Bedard (4-7), just off the disabled list (knee) and the subject of trade rumors, showed the effects of his 32-day layoff. He threw 57 pitches in just 1 1/3 innings and was tagged for five runs and three hits, walking four and striking out two.
The Rays sent a season-high 12 batters to the plate in the second. The eight runs are the most since June 25, 2008 when the Rays scored 10 in the fifth inning against Florida.
The Rays also scored seven run in the seventh inning Thursday against Oakland. It's the first time the Rays have scored seven or more runs in an inning in consecutive games.
Bedard and Aaron Laffey combined to walk four in the inning and all four scored.
Bedard issued a bases-loaded walk and gave up a two-run single to Johnny Damon. Laffey yielded Zobrist's two-run shot on a 2-1 pitch.
"We did a really good job of making him [Bedard] come into our zone, especially those pitches high in the zone and curves down in the zone," Zobrist said. "We did a good job of zoning into our pitch and not chasing. Whenever you do that it makes it harder on the pitcher. He's really got to bring strikes early in the count and he just wasn't doing that."
The Mariners have lost 22 of their past 27 games, with a franchise-record 17 straight losses during that span. They are 5-19 in July with two games remaining. Their worst July in the franchise's 34 seasons was last year at 6-22.
It's the ninth shutout for the Rays this season and the ninth time the Mariners have been shut out.
"He did a good job with his fastball and a couple different types of breaking balls," Mariners manager Eric Wedge said. "He threw some good changeups to left handers. He threw the ball well tonight."
Five Mariners pitchers issued seven walks, matching the season high set July 20 against Toronto.
- Before the game, the Mariners designated DH Jack Cust for assignment to make room for Bedard. "He's a class act. He was good in the clubhouse, good teammate, hard worker," Wedge said. "He just really wasn't able to get it going for us this year." Cust, 32, who signed a $2.5 million, one-year deal, hit just .213 with three home runs and 23 RBI in 67 games.
- Wedge is considering going with a six-man rotation to help preserve the young arms on the staff.
- Maddon said he talks to GM Andrew Friedman every day on possible trades. He said, "our names are bandied about every year and nothing ever happens. So I'm prepared for nothing happening."