CLEVELAND — Joe Kelly could be a reliever already. He could have taken his 98 mph fastball to the bullpen long ago.

But Kelly – whose start was postponed Thursday when Boston’s game at Cleveland was rained out – instead came out of spring training as the No. 3 starter in the rebuilt Red Sox rotation, a starter perhaps poised to be the difference-maker Boston hoped he’d be when he came over from St. Louis in a trade for John Lackey two years ago.

That he uses everything in his repertoire beyond that fastball is what gives him a chance to do that.

“I’d heard all about the fastball,” Red Sox pitching coach Carl Willis said. “When I saw his first bullpen and I saw the secondary stuff, I was like, ‘Wow, this isn’t just a guy that throws 98 miles per hour.’ ”

“His ability to command his secondary pitches has been exceptional,” said catcher Ryan Hanigan.

Kelly carried a 5.67 ERA when the Red Sox shipped him to Triple-A Pawtucket in late June last season, driving home the message he needed to use his secondary mix far more than he had. Six successive starts in May and June had seen Kelly throw his fastball more than 69 percent of the time, just rearing back and firing, reliever-style. It hadn’t worked.

In his final eight starts of the season, all after his recall from Pawtucket, he threw his fastball more than 60 percent of the time just once and less than 50 percent three times. He posted a 2.35 ERA in those eight starts. He carried that over into spring training, pitching to a 2.63 ERA in 24 innings.

A snowy but efficient bullpen session Monday was one last opportunity for Kelly to solidify his feel for his pitch mix.

“Everything feels like it’s coming out of the same arm slot,” he said. “The command of the secondary stuff has felt pretty solid, getting over the plate for some strikes and then burying some stuff when I needed to.”

Kelly made clear late last season that he was leaving his game-calling in the hands of his catcher. The way Hanigan guided him through most of his starts convinced Manager John Farrell to pair Kelly with Hanigan this season, at least in the early going.

But Kelly does understand the value of giving Hanigan as many options as possible. What makes him so intriguing as a starter is the myriad of options his four pitches give his catcher. Those options give him a better chance to turn over a lineup three times than a pitcher with just two or three pitches.

One new pitch Kelly unveiled this spring, for example, was a hard slider he can throw in under the hands of left-handed hitters, almost like a cutter.

“It’s the same pitch,” he said, “but sometimes I’ll put a little extra on it if I feel like it’s good for that hitter, and sometimes I’ll try to slow it down if a guy is on it.”

“It’s always nice to have the option of a pitch they haven’t seen when you get through the lineup the third or fourth time,” Hanigan said. “He can throw a righty-righty change-up. Not a ton of guys do that. He can throw a backdoor slider. He can throw a frontdoor slider. He’s one of the best guys at throwing get-back-in-the-count breaking balls or strike breaking balls in fastball counts. He can throw the four-seam heater and the two-seam heater. Really, it’s about thinking it and being smart with the game-calling.”

There was a time Kelly might have preferred pitching out of the bullpen, but since Boston got him, he’s wanted to do nothing but start. That the Red Sox targeted him in the Lackey deal showed they believed he could start.

“It’s obviously been a challenge,” he said, “but I’m trying to learn year to year about what it takes. It’s been fun just to try to learn that stuff on the fly.”