(CBS) Before suffering a setback with a hamstring strain recently, Cubs right-hander Jake Arrieta had returned to the form of an ace once again.

Arrieta has bounced back from a slow start to the season that saw him have a 5.44 ERA in mid-May. He’s now 14-9 with a 3.48 ERA and showing his form from the past. Monday brought a Arrieta’s run to a halt as he suffered a Grade-1 hamstring strain, but he’s only expected to miss one or two starts.

Cubs president of baseball operations Theo Epstein is eager to get Arrieta back after watching how he has returned to prominence with his pitching. Speaking Thursday on the Bernstein and Goff Show on 670 The Score, Epstein explained why Arrieta has looked dominant as of late.

“With Jake, I think so much of his success relies on getting to a point where he locks in his delivery, because it’s not a traditional delivery,” Epstein said. “Where having the right timing, the right balance and the right rhythm in his delivery is so important.

“That slider’s starting to come back, his fastball started moving all over the place, the velo picked back up a little bit, and he got in such a good rhythm, that he felt like he was going to win every time he takes the mound.”

Arrieta’s greatest struggles of 2017 came early on. He struggled to go deep into games due to troubles with his command.

For Arrieta, success relies on that tenacious slider and velocity to create a different look with his repertoire. That’s finally back in play.

“It’s a feel thing for him and a lot of guys throwing across their body to that extent,” Epstein said. “He worked extremely hard to get to the point where he could lock it in and did. And the great thing about Jake is that even when he didn’t have his best stuff – didn’t have his slider – he didn’t back down, always ultra-competitive with what he had. Now that he’s got premium weapons back, it’s been fun to watch the roll that he’s on. “

Epstein and the Cubs will have to decide whether the 31-year-old Arrieta can remain that same pitcher on his next contract. He will likely receive heavy interest around the league when he becomes a free agent in November.

For now, the Cubs are focusing on getting Arrieta back into their rotation. He’s expected to miss one or two starts but should be able to return later in September as the Cubs look to close out the NL Central.

“The key now is just getting him back on the mound,” Epstein said. “He felt pretty good yesterday. We’ll continue monitoring him day-to-day. But we expect to get him back in time to make an impact in the regular season and hopefully beyond.”