Corey Kluber's return to the New York Yankees rotation was questionable after being sidelined in late May due to a shoulder strain, but the two-time AL Cy Young Award winner said Friday he is "very confident" he can return this season.

"[I am confident] because of the way I've felt throwing, the way I bounce back, the calendar, things like that," Kluber said before the Yankees' 6-2 loss to the Boston Red Sox at Fenway Park. "The way I look at it is that I'm not really trying to think I'm up against a timeline because I think the natural process of the rehab will allow for that."

Yankees manager Aaron Boone said Kluber threw 25 pitches Friday in his first mound session since being placed on the 60-day injured list, calling it "a good step for Corey."

In his first outing following his first career no-hitter in May, Kluber was pulled after throwing three innings against the Toronto Blue Jays. An MRI showed a subscapularis strain of his right shoulder, which led to his current stint on the injured list.

Kluber's shoulder strain required full rest for at least four weeks, followed by at least four weeks of rehabilitation. And though he will be eligible to come off the IL on July 25, Kluber avoided offering a specific timeline for his return.

Kluber described his throws on Friday as "easy fastballs," adding that it was "nothing like I would do in a regular bullpen session."

"I have to sit back and wait for the process to unfold," he said. "I've just found it's easier for me to take it day by day and not get ahead of myself, not look too far in advance. I think that's probably the best way ... It's about paying attention to the way you recover and the way you bounce back when there is a new step along the way."

The 35-year-old Kluber had worked his way back from injury to earn his place as the No. 2 starter in New York behind Gerrit Cole. Kluber went 4-2 with a 2.86 ERA and 50 strikeouts before the injury.

Also Friday, catcher Gary Sanchez was taken out of the game in Boston with what the team described as "mid-back spasms."

With Cole's personal catcher, Kyle Higashioka, still on the COVID-19 IL, Sanchez caught Cole for a second straight start. The Yankees ace threw five innings and was tagged with his fifth loss of the season after allowing three runs on six hits, including Rafael Devers' first of two home runs. The Cole-Sánchez battery held the Red Sox to one run in six innings in the Yankees' rain-shortened victory at Yankee Stadium last Saturday.

"We'll see how he responds overnight," Boone said about Sánchez's availability for the rest of the series. "He's in there getting treatment now, and we'll see where we're at tomorrow. It's just something that locked up on him and hopefully it's a short-term thing."