The Saints grabbed the first of three tickets to the second tier of English football with a 2-0 home defeat to Southampton on Saturday.
The defeat brought to an end an 11-year stay in the English top flight.
Southampton have lost 24 league matches this season, the most in a single campaign by the Saint Mary's outfit.
Selles is the third manager Southampton have turned to this season but none of them could inspire the needed changes.
“It’s a tough day for everybody, the performance on the pitch was not what we expect it to be and we were not good enough today and that’s why we are in this situation,” said the Spaniard.
“If we were in our standards, we would not be talking about this situation right now. We need to face it like that.
“I don’t think there is one point where you can say that is exactly the point where it happened.
“As a club we need to evaluate and see what the standards that James referred to are and be sure that when the club starts the next season those standards are on point.”
“I take all the responsibility for the last three months, that’s my responsibility,” he added.
“I should be able to win more football matches. I should be able to have a team that competes better.
“I think I could have done (things) different; more is difficult but I think I can do different. I made my mistakes like any other and of course it was not good enough.”
