Team selection limits

[arthur smith]

iv 2 matches today, 16mins apart, 1st is league, 2nd is CL final against a team quite a bit better than me. So was going to play a lesser team in match one to avoid injuries/fatigue etc then quickly change it for the final we the best team. First match takes about 5 mins to run so do I have adequate time to alter the team for final or will I have to put final team on in game 1 and run the risks?

Previously I have tried to alter my team with maybe a few minutes to go to KO and get the message I cannot as game starting but cannot find anywhere what the limit to change team prior to KO?