Both players suffered injuries during Monday night's win over St. Louis.
Hixon was having tests, including an MRI, on what Coughlin thought was a knee injury. Manningham was following NFL protocols on testing for a concussion.
A team spokesman said the team might not have updates for either player until Wednesday.
Hixon's right knee seemed to buckle on a juggling touchdown catch in the second quarter, although the wide receiver said afterward that it was a calf injury.
Coughlin said Manningham "was much clearer" when he left the stadium following Monday's game and seemed even better Tuesday.
"He was much more bright-eyed this morning when he was in, but nevertheless, he's going through the entire battery of tests that are part of the protocol for someone who leaves the game with any type of head injury," the coach said.
The banged-up Giants play at NFC East rival Philadelphia on Sunday.