The 24-year-old came through the ranks at Fratton Park and made 41 appearances for Pompey after making his debut in 2016.
Bass was loaned out to League Two side Bradford in the second half of last season and made 21 appearances for The Bantams, before signing a three-year deal on Wearside, with a club option of a further year, for an undisclosed fee.
To find out more, we caught up with Portsmouth writer Jordan Cross at our sister title The News to get his thoughts.
What’s Bass’ background at Portsmouth?
JC: “He’s got quite a backstory with Pompey, he’s an academy graduate and has been at the club since he was a young teenager.
“He was given his debut in 2016 at the age of 18 and was quite highly thought of within the club with hopes that he would become a long-term first choice here.
“There were bits and pieces of first-team involvement after that until January 2020 when Kenny Jackett sprang a bit of a surprise. Craig MacGillivray was the established number one and then he brought Alex in and he made 25 appearances that season.
“Crystal Palace were showing interest in him because he was coming through quite strongly.
“It looked like he was going to start the next season as first choice but then had a really ropey game at Stevenage in the cup and Jackett went back to Craig MacGillivray.
“That was the beginning of some problems for Bass, he broke his leg in January 2021 in training and was out for about seven or eight weeks.
“He went out on loan to Southend and rebroke it and suffered a recurrence of a fractured fibula, and admitted he feared his career might be over.
“By the time he came back Kenny had gone and Danny Cowley had come in, and he had to make a choice on goalkeepers without really having a lot to go on with Alex.
“He made the choice to go with Gavin Bazunu, who was a revelation last season, which worked very well, and Alex went out on loan to Bradford in the second half of last season.”
Why have Portsmouth decided to let him go?
JC: “It became quite clear that he didn’t quite fit with Danny Cowley’s game idea, especially when there was a lot of emphasis on keepers being like Gavin Bazunu and being competent with their feet.
“Danny made the choice that wasn’t one of Alex’s strong points, which I think was perhaps a little hasty, and Alex does as well. He believes it’s something he can do and has shown that in the pre-season before Josh Griffiths’ arrival.
“I think by that stage the decision had been made to go with another young keeper on loan, try to sell Alex and get some money to work with quite a tight budget.”
Was there much interest in him from other clubs?
JC: “Crawley were staying in the same hotel in Spain as Portsmouth, so we got wind that something was in the offing there.
“When Pompey then signed Josh Griffiths, I think Danny then made it clear to Alex that he was going to be free to sort himself out.
“Crawley was the first team to emerge when they were away, a few days later it was Exeter, then we got whispers of Sunderland at the weekend.
“It looked like it would be Exeter where he went but then the Sunderland stuff really emerged and he was up on Wearside putting the finishing touches to a deal.”
What can Sunderland fans expect from Bass?
JC: “He’s very dependable and competent with his handling, a really good shot stopper.
“He’s a good size, maybe about 6 ft 3, and maybe has a bit more filling out to do.
“He had some good goalkeeping coaches at Portsmouth, particularly John Keeley who thought the world of him.
“I know he spoke to Crystal Palace and had meetings with Crystal Palace when he broke through.
“Even last season he played away at Fleetwood and was excellent, he made two league appearances last season and was man of the match on both occasions.
“Wycombe was another game, and we all know Wycombe can be a tough game for a goalkeeper, but he stood up to it really well.
“He’s a really good well-rounded goalkeeper.”
How has the news gone down with Portsmouth fans?
JC: “I think Danny Cowley was aware of that and was eager to get out that it was a good deal for the club financially.
“When you lose a homegrown youngster that’s always a disappointing one for supporters.
“I think everyone had accepted that he was good enough to be a first-choice keeper and play at League One, or drop down a level to get those minutes.
“There was a disappointment among supporters but I think there was an acceptance that he had to go somewhere and get his career going.”
Do you think he’ll be a good signing for Sunderland in the Championship?
JC: “Absolutely, even if he doesn’t go in as first choice.
“Sunderland are a big club, they are in the Championship and it’s a three-year deal, that’s all appealing but he wants to play football.
“There’s a gap there, but perhaps he’s not a million miles away, he’s perhaps closer than it seems to being first choice.
“I’m sure he will go in there with ambitions to become first choice.
“There will certainly be a lot of people at Portsmouth willing him on to do that this season.”