Tottenham had to dig deep as goals from Shane Long and Danny Ings cancelled the hosts’ early lead.
Spurs were heading out of the competition with less than 20 minutes remaining, but Lucas Moura levelled before Heung-Min Son booked a home tie with fellow Premier League opponent Norwich City next month from the penalty spot.
Speaking to BBC Sports after the game, Mourinho conceded the Saints were the better team and blamed fatigue for his team’s sluggishness.
"The best team lost. The best team on the pitch lost,” the former Chelsea and Manchester United boss said.
“If you look at it in a pragmatic way, forgetting that they had their best players, they had their full team, they had one more day to rest, we were really in trouble.”
"We were in real, real trouble to build a team, lots of fatigue after the match against Manchester City.”
“If you forget these important items and you analyse in a simple pragmatic way, the best team lost.”
"They were better than us, they were faster than us, and they were stronger physically than us. They press, they had intensity, and they had chances.”
"They were the best team, but I cannot forget all the other circumstances, and my players, they were phenomenal.”
“The way they react was fantastic. They showed incredible heart and had a great connection with the crowd.”
"They gave absolutely everything. I think the best team lost, but we deserved to win."
Tottenham have progressed from each of their last 11 replays in the FA Cup, with six of those wins against top-flight opposition.
