This game cannot be flexed out now. NFL has to make decision no less than 12 days in advance of the game, per their flex guidelines. That was December 11.
NFL has already finalized schedule for this weekend.
Week 16 (December 23) Schedule Change: Giants-Colts Moves to CBS
FOOTBALL INFORMATION 2018 REGULAR WEEK 16 ENGLISH
The NFL announced today the following schedule change for Week 16 (December 23):
The New York Giants at Indianapolis Colts game originally scheduled for 1:00 PM ET on FOX will move to 1:00 PM ET on CBS.
The following is the final Week 16 NFL schedule (all times ET):
Saturday, December 22
Washington at Tennessee
4:30 PM
NFLN
Baltimore at Los Angeles Chargers
8:20 PM
NFLN
Sunday, December 23
Atlanta at Carolina
1:00 PM
FOX
Cincinnati at Cleveland
1:00 PM
CBS
Tampa Bay at Dallas
1:00 PM
FOX
Minnesota at Detroit
1:00 PM
FOX
New York Giants at Indianapolis
1:00 PM
CBS
Jacksonville at Miami
1:00 PM
CBS
Buffalo at New England
1:00 PM
CBS
Green Bay at New York Jets
1:00 PM
FOX
Houston at Philadelphia
1:00 PM
CBS
Los Angeles Rams at Arizona
4:05 PM
FOX
Chicago at San Francisco
4:05 PM
FOX
Pittsburgh at New Orleans
4:25 PM
CBS
Kansas City at Seattle
8:20 PM
NBC
Monday, December 24
Denver at Oakland
8:15 PM
ESPN