London - Graeme Souness believes Manchester United face a lengthy transition period if manager Louis van Gaal's favoured 3-5-2 formation is to succeed at Old Trafford.
Van Gaal has still to oversee his first Premier League win as United boss, with the faltering English giants having lost to Swansea on the opening day and been held to draws by both Sunderland and Burnley.
Meanwhile the Dutchman also suffered the embarrassment of seeing United knocked out of the English League Cup following a 4-0 defeat by third-tier Milton Keynes Dons.
Liverpool and Scotland midfield great Souness questioned whether United had the players in their squad to set-up with three at the back.
"If Van Gaal is determined to persist with a back three, this is going to be a long and painful transition for United," the former Liverpool, Rangers, Blackburn and Southampton manager wrote in his Sunday Times column.
"The team may take until November to settle down. You ain't going to win the league if you take that long and you might not make the Champions League places either," Souness added.
"United have already dropped seven points from a possible nine. If you continue with three at the back and the results are not improving, players will not be slow in turning round and pointing their fingers at the coach, saying, 'It's not that I am not playing well, it's the system'."
Souness also questioned whether the system, which Van Gaal deployed successfully while overseeing his native Netherlands' run to third place at the World Cup in Brazil, would suit United's British-record signing Angel di Maria, who joined from Real Madrid for 59.7 million ($98 million, 75 million euros) last month.
"Angel di Maria is too much of a winger to play as a wing-back, so he could play one position inside the widest man in a 3-5-2," said Souness.
"If you play with a back four you can play him wide, as Argentina did when he made three goals and scored the other in their 4-2 win over Germany in midweek.
"I don't read much into international friendlies but that performance showed his talent."
United will go in search of their first Premier League win under Van Gaal when they face Queens Park Rangers at Old Trafford a week on Sunday.