The Red Devils have been heavily criticised for their performance in Tuesday night's Champions League last 16 second leg against Sevilla at Old Trafford.
Throughout the season, big-name players including Paul Pogba have been criticised for not playing at a consistently strong level, and the same can be said for January arrival Alexis Sanchez, who has scored once in 10 outings.
"If you were to look at the players who were on paper, there are some good players in there," Souness told TV3. "But from Alexis Sanchez to Paul Pogba, who's really playing to their full potential? And that has to come down to the manager.
"The manager isn't getting the best out of the group and that's your job. It's not happening for United right now."
Mourinho's only chance of guiding the club to silverware this season is winning the FA Cup.