Happ is fully recovered from the same disease that afflicted New York Mets right-hander Noah Syndergaard recently and is expected to pitch Thursday night in the opener of a four-game series with the Texas Rangers.
"Good news. He is out of the woods virus-wise and not contagious," Yankees manager Aaron Boone said Monday, two days after Happ was supposed to start in Boston.
Happ will be making his second start since New York obtained him July 26 from the Toronto Blue Jays for infielder Brandon Drury and outfielder Billy McKinney.
The left-hander's debut was an impressive one as he allowed one run and three hits in six crisp innings during a 6-3 win over the Kansas City Royals on July 29. Happ is 11-6 with a 4.05 ERA overall this season and will be making his 250th career start.
Happ will be making his second start for the Yankees under unique circumstances. When he made his debut, he flew in from Chicago the previous day after being delayed by weather.
This time, Happ will be starting after spending the last week in New York recovering from a disease known mostly for afflicting children.
The Yankees used rookie Chance Adams in Happ's place last week, and he allowed three runs in five innings in a 4-1 loss.
Meanwhile, Happ is hoping to pick up where he left off from his debut with his sixth team when he averaged 92.9 mph on 64 four-seam fastballs.
"You definitely want to have good results," Happ said after his Yankee debut. "They traded for you. There's some pressure in that."
Happ last faced the Rangers when he allowed two runs and five hits and struck out nine in seven innings in Toronto on April 29. He is 4-2 with a 2.97 ERA in six career starts against Texas.
When Happ won his debut, the Yankees ended the day 5 1/2 games behind the Boston Red Sox in the American League East. Now they are nine games out of first place following a 3-4 trip that ended with Wednesday's 7-3 win over the Chicago White Sox.
Giancarlo Stanton hit a grand slam in the second inning and has 18 RBIs in his last 18 games.
While the Yankees are playing mostly teams under .500 until visiting Oakland on Sept. 3, the Rangers are playing better of late. They are 9-3 in their last 12 games after going 2-11 in the previous 13 games from July 9-26.
Texas also is 24-21 since June 16 when it followed up a seven-game losing streak with a seven-game winning streak.
The Rangers head to New York after Joey Gallo homered twice and their offense totaled a season-high 19 hits in Wednesday afternoon's 11-7 win over the Seattle Mariners. Gallo hit his 30th and 31st homers as part of a 3-for-5 showing and is hitting .279 (17-for-61) with nine homers in 16 games since the All-Star break to raise his batting average from .187 to .202.
"I think my swing is starting to develop a little bit better," Gallo told reporters. "I've always had good hands and fast hands, but I used to have some hitches in my swing that wouldn't let me get to it. My angle to the ball, it wouldn't let me get to it. Now, my swing is kind of leveling out when I'm doing well, so obviously that just makes it easier to hit that inside pitch, that up-and-in pitch."
Rougned Odor contributed two hits and is hitting .345 (10-for-29) in August.
Texas will open the series by starting Ariel Jurado, who is making his fourth career start. He improved to 2-1 and lowered his ERA to 4.02 when he allowed two runs on six hits in five innings of an 11-3 win over the Baltimore Orioles on Friday. He has won his past two starts.
Jurado is the seventh native of Panama to appear for the Rangers and is looking forward to pitching in the same place where fellow Panamanian Mariano Rivera starred.
"This is an honor for me to pitch in a stadium that is so famous," Jurado told reporters through an interpreter. "For me, I'm going to continue the same things I'm doing."
Texas took two of three at home from the Yankees on May 21-23 when it scored 18 runs in its two wins. The Rangers are 7-3 in their last 10 visits to Yankee Stadium since the start of the 2015 season.