FOXBORO, Mass. — The New England Patriots’ offensive line depth is quickly diminishing.

Guards Tre’ Jackson, who is on the physically unable to perform list, and Jonathan Cooper and center Bryan Stork were absent from Day 4 of training camp Sunday. Wide receiver Keshawn Martin was newly absent, and running back Dion Lewis, also on PUP, hasn’t been spotted yet this summer.

Cooper suffered a plantar fascia strain Saturday during practice, a source told NFL Media’s Ian Rapoport.

Guards Josh Kline and Shaq Mason haven’t been full participants in practice, leaving rookies Joe Thuney and Ted Karras on the top offensive line unit.

Wide receivers Julian Edelman, Danny Amendola and Nate Washington, offensive tackle Sebastian Vollmer, tight end Clay Harbor and defensive tackle Alan Branch were present but didn’t participate and weren’t wearing pads. Edelman, Amendola, Vollmer and Harbor are on PUP, and Branch is on the non-football injury list.

Cornerback Logan Ryan and safety Duron Harmon continued to wear red non-contact jerseys.

Wide receiver Malcolm Mitchell, defensive tackle Vincent Valentine and running back Brandon Bolden returned to practice after being absent Saturday.

Thumbnail photo via Mark J. Rebilas/USA TODAY Sports Images