1Co 11:25 - "In the same way the cup also, after supping, saying,"

Luk 22:20  "And in like manner the cup, after having supped, saying,"

It is not after supper but Jesus supped a mouthful from it and then said.....

1Co 11:25:

"In the same way the cup also, after supping, saying,"

Luk 22:20:

"And in like manner the cup, after having supped, saying,"

Conclusion:

It is not after supper or after the meal (in this verse) but Jesus took the cup, supped a mouthful from it and then said.....