Mousinho left his hybrid player-coach role at United in January 2023 to take over at Fratton Park, and led Pompey to the Sky Bet League One title during his first full season.
However, life in the Championship has proven difficult so far for the south coast club, and they are yet to win any of their opening eight games, the only team in the division not to pick up three points yet.
Pompey have drawn four and lost four, leaving them second from bottom in the table, above Cardiff City on goal difference.
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The hammering suffered at the Potters now puts huge pressure on Pompey ahead of the U’s visiting Fratton Park on Saturday afternoon.
A brace from Leicester City loanee Tom Cannon, plus an effort from Sam Gallagher, gave the Potters a 3-1 lead at the break, with Mark O’Mahony equalising from Cannon’s opener for Pompey.
Three goals in five second half minutes almost immediately after the break blew Pompey apart though.
Cannon added another two goals, including one from the penalty spot, while Brighton & Hove Albion loanee Andrew Moran completed the scoring eight minutes into the second period.
Pompey head coach Mousinho told The Portsmouth News: “Myself and the coaching staff are taking full responsibility for anything we need to take, but the players have to take a long, hard look at themselves as well. It’s their careers that these sorts of performances affect.
“I didn’t see this coming, it was a big surprise to me because we had been so good in the last three games. We were excellent at the weekend against promotion contenders and had a real chance to come and get something.
“It was the nature of the way we went under. We didn’t run, I didn’t think we competed, we lost tackles, we lost headers, we lost second balls. The players have to do a lot better than they did.
“I’ve learnt loads, I’ve learnt loads about individuals. We’ve learnt that certain moments, when the going gets tough, I thought we saw players go under. In general, if you look at the way we started the game, we have too many players making the wrong decision.
“I’m not entirely sure where that comes from. You come to Stoke and decide we’re going to allow them to build pressure in the first 10 minutes and allow them to get their heads up and allow them to score a goal. It’s beyond me.
“It is probably too early to say whether we’re in a relegation battle, but we’re not exactly covering ourselves in glory in terms of picking up points.”