Lucas Moura struck to bring the home team level before a Son Heung-min penalty booked a clash with Norwich City in the last 16. "To go from such a big game at the weekend [a 2-0 defeat of Manchester City], it's easy to lose a game like this".
Mourinho added: "To arrive here we had to play four matches, really hard".
The move means Wanyama, who has not been selected by Mourinho to play for eight straight matches, is only available to play in the English Premier League and FA Cup. Dele changed the game after we switched to a back four.
Dele then started pulling the strings.
The 20-year-old made his Premier League debut against Liverpool, and produced a solid performance. "Victor is from a position (midfield) where we have lots of players - (Harry) Winks, (Eric) Dier, (Oliver) Skipp, (Gedson) Fernandes".
"So many difficulties to even build a team, the options were not options to change the game". Incredible soul, incredible heart, and they gave absolutely everything. I'm so happy for them. "Yes, I think so", Mourinho explained.
The likes of Charlton Athletic, Queens Park Rangers, ADO Den Haag and Burnley were all interested in signing the striker in January, but it is unknown whether any will choose to reignite their interest going forward.
"They were the best team, but I can not forget all the other circumstances, and my players, they were phenomenal".
"One game we have players A, B and C out and then the next game we are without D, E and F. It's been a very hard season". Maybe we need that match for something.
Southampton had 57% of possession, had 15 shots compared to nine from Tottenham and also had seven attempts on target with the hosts scoring from all of their three goal-bound chances.
"I think the best team lost - but we deserved to win".
"It has been a long time coming, we've been one of the best teams in the world in the years we've been together, we deserve a trophy".
Matches in that round will not be replayed, so if the Spurs-Norwich contest is tied at the end of regulation extra time, penalties will determine who reaches the quarterfinals. 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.