Mourinho also indicated he could hand 20-year-old goalkeeper Joel Pereira a start in their final Premier League game of the season against Crystal Palace on Sunday.
His side will have played 64 matches this season once the Europa League final is done but Souness has no sympathy for Mourinho or his United players.
De Gea is the subject of a potential £60million move to The Bernabeu although Mourinho will battle to keep him.
United played their 62nd match of the season last night as a much-changed line-up drew 0-0 at Southampton and by the time they play Ajax on May 24 in the Europa League final they will have had a 64-match season.
"We have the Argentina national goalkeeper and the Spanish national goalkeeper: two of the best national teams in the world with good traditions of goalkeepers in both countries".
It's a tremendous effort but [there has been] no help unfortunately from the Premier League to give him the best chance of winning a trophy that Manchester United haven't won. You know that in all of my career I was never out of European competitions in the group phases. Tells (Wilfried) Zaha to go slow, leaves (Christian) Benteke at home. "They showed me they want to be an option for the final". I never saw that with Chelsea, I never saw that with Manchester United, I never saw that with Manchester City a year ago.
Kapil Mishra attacked in house
Police have received a complaint from Mishra and are taking appropriate legal action, said DCP (North) Jatin Narwal. I've known Kejriwal ji for last 15 years and Kapil Mishra for almost 10 years when the RTI movement had started.
Even his return to Stamford Bridge in 2013 saw Mourinho possess a more reserved personality, and he called himself "the Happy One" when first introduced to the media.
Southampton have now gone four top-flight home games without scoring for the first time and remain eighth, which is the highest position they can hope to achieve ahead of Sunday's final game at home to Stoke City.
"Sergio had a very good performance, I would say more than very good", added Mourinho.
But Mourinho was surprised with this news as he says he fully expects the Spaniard to line up for United in their opening 2017-18 pre-season game against LA Galaxy on July 15th.
Southampton struggled to relieve the pressure building on manager Claude Puel against an under-strength United.