ST. LOUIS — The Cubs added a newly repaired reliever and subtracted an old reliable one Tuesday as they continued to seek upgrades in the final hours before Wednesday’s trade deadline.

Four days after acquiring left-hander Derek Holland from the Giants, the Cubs traded pitching prospect Thomas Hatch to the Blue Jays for right-hander David Phelps — who returned from multiple surgeries, including Tommy John surgery, only last month.

The Jays also included cash in the deal.

Phelps, who didn’t pitch from Aug. 31, 2017, until June 17 this season, has a 3.63 ERA in 17 appearances for the Blue Jays.

Phelps, 32, last pitched Sunday against the Rays, allowing three runs on two homers and recording just two outs.

The Cubs also put longtime setup ace Pedro Strop on the injured list before Tuesday’s series opener against the Cardinals in St. Louis because of a tight left side of his neck — which Strop said developed Saturday night into Sunday, when he tried to play catch.

Strop has been on the IL twice in the last year for hamstring injuries, and his velocity has dipped 2-3 mph recently.

Duane Underwood, who made his big-league debut with a four-inning start (allowing one run) against the Dodgers last season, was added to the bullpen Tuesday night.

The Cubs transferred left-hander Xavier Cedeno (wrist) to the 60-day injured list to make room on the 40-man roster for Phelps.