The Spaniard, who was appointed to succeed Nathan Jones when the Welshman was sacked in February, has been unable to keep Saints in the Premier League.
After picking up seven points in the first four league games under Selles, Saints have not won since and were confirmed as relegated with two matches spare.
Saints have now confirmed that Selles will depart the club at the end of the season, with Swansea City boss Russell Martin reportedly poised to take over.
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Ward-Prowse, however, leapt to the defence of Selles – taking on his first job in senior management – in an interview with BBC Radio Solent.
“The period we’ve had under Ruben has been really enjoyable, regardless of what people look and see or feel from the outside, I can tell you from within it,” the 28 year old said.
“He’s been fantastic for us since day one, he walked into the job like he’s been in it for five or six years – it will be an emotional day to see the season off in a disappointing way for the first time in my career.
“We as a squad owe it to Ruben as he’s been fantastic, we want to give him everything we can as a good send-off if you like.”
Ward-Prowse added: “It would be unprofessional to be thinking about anything other than the game, everybody is playing for something – the next manager, whoever that will be, will be watching the game and assessing the players ahead of taking over the role.
“I think the big word from me over the season has been consistency, from a playing perspective we picked up good results versus good teams and failed to follow that up with a string of results.
“Off the pitch, the decisions have taken its toll on the group and a combination of a lot of things have led to the situation.”