Tight end Dwayne Allen and offensive lineman Justin Anderson both sat out Indianapolis' practice Thursday morning.
Allen is out with a foot injury that was still being evaluated.
But Anderson's injury appears to be more severe. Coach Chuck Pagano said he had a "significant" pectoral muscle injury and early indications were that he may need season-ending surgery.
Both were Colts draft picks in 2012.
Last year, Allen, a third-round pick, was the NFL's receptions leader among rookie tight ends with 45. He broke Ken Dilger's franchise rookie record, too.
Anderson, a seventh-round pick started last season on the PUP list, was cut in November and then re-signed to the Colts' practice squad.
Rain and light hail forced Thursday's practice onto the turf field in Anderson. The Colts will hold a public practice Friday at 1:50 p.m., then take Saturday off before hosting Buffalo in the preseason opener Sunday at Lucas Oil Stadium.
(Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)