Ortega has played in every round during City's run to Wembley and will keep his place in the team against United at the expense of usual No. 1 Ederson, who is set to return to the XI for the Champions League final against Inter Milan in Istanbul on June 10.
"He is going to play," Guardiola said, when asked about Ortega at a news conference on Friday. "I have always been like that in the FA Cup. At Barcelona and Bayern as well. The goalkeeper who plays in the FA Cup is going to play."
Ortega's inclusion in City's team opens the door for Guardiola to stick with other players who have been rotated into the XI for domestic cup games.
Aymeric Laporte and Riyad Mahrez have usually been substitutes in the Premier League and Champions League during the second half of the season but have played in the FA Cup. Mahrez scored a hat trick in the semifinal victory over Sheffield United at Wembley in April.
"They trained well the last two sessions," Guardiola said. "They are all of them more or less fine. It's the final of the FA Cup. It's a pleasure to be here, we will travel tonight [Friday] and be ready for tomorrow. It's a final, nothing else."