Decker injured the muscle Wednesday and saw limited action Thursday.
"I think Demaryius has had a really good camp," Manning told reporters Thursday. "Demaryius is a guy that we're going to feature."
Manning, entering his 15th season in the NFL, noted that Thomas, listed at 6-foot-3 and 229 pounds, is the biggest target he has had in his career.
"He's the biggest receiver I've played with since I've been in pro ball, certainly," Manning said. "He has that rare combination of size and speed. So, I look forward just to getting more and more comfortable with him and just kind of growing through this process."
Thomas told Around the League's Dan Hanzus last month that with a Hall of Fame-bound quarterback like Manning on the field the pressure is on to do everything correctly.
"Demaryius, these past two days, has really made some big plays," Manning said. "Done some big things with the ball after the catch, which what we're going to need. You can't just throw the ball down the field every time. You've got to take a five-yard hitch or a 10-yard hook route and turn it into a 20- or a 30-yard gain, and he's kind of shown us those type of plays."