Craig Counsell’s most used — and one of his most trusted — relievers is headed for the IL.

The Cubs placed Mark Leiter Jr. on the 15-day IL with a right forearm strain Sunday evening and recalled lefty reliever Luke Little.

Leiter, 33, tallied 28.2 innings in a team-high 32 appearances. Early on this season, Leiter was the Cubs’ most reliable bullpen arm, at a time when the Cubs were struggling to piece together their reliever corps. Leiter had tallied 12.1 scoreless innings to start the season and had a sub-1.00 ERA as recently as May 21.

After that point, he allowed 7 earned runs over his next 2 innings of work across 4 outings. He followed that with 4 straight scoreless appearances but had struggled over the last 12 days. In 4 appearances since June 11, Leiter had allowed 8 earned runs in 3.1 innings. The Giants scored 3 runs on a hit and 2 walks in 0.2 innings in his last outing on Wednesday.

That appearance against the Giants came at a time when the Cubs’ other most trusted arm was struggling. Héctor Neris had blown a save two days earlier that cost the Cubs a win and he had another blown save the week prior in Tampa Bay.

Since Neris’ blown save, Counsell has turned to Keegan Thompson and Colten Brewer to pick up saves against the Giants. Now, Counsell will likely turn to that duo, along with Neris and Tyson Miller in leverage situations.

Leiter was a revelation for the Cubs in 2023.

The righty tallied 69 appearances out of the Cubs’ bullpen, amassing 28 holds and 4 saves. He was part of the Cubs’ three-headed bullpen monster with Julian Merryweather and Adbert Alzolay at the end of games last season.

Now, all three are on the IL, as Merryweather (rib stress fracture) and Alzolay (right flexor strain) have combined to miss 108 games this season. Yency Almonte, an offseason acquisition who was developing into a key bullpen piece, has missed 38 games with a shoulder strain, too.

Little, 23, was optioned to Iowa on Saturday when the Cubs activated Thompson off the paternity list. He remained in Chicago and is available out of the bullpen for Sunday’s finale against the Mets.

José Cuas — who was designated for assignment a week ago — was claimed off waivers by the Toronto Blue Jays.