The Reds have won only one of five ties at Old Trafford this season in the Champions League and Europa League, against Otelul Galati, and conceded an average of two goals per game.
It's a far cry from last season's imperious form at the Theatre of Dreams, both domestically and in Europe's premier competition, and the manager will be assessing the situation.
"I think there’s been that slackness all season [with defending]," he told MUTV. "And, but for David De Gea, we could have lost by four or five goals [to Athletic]. He was absolutely superb.
"We’ve had a bad season in Europe, it has to be said. We’ve lost three goals against Basel, two against Benfica, two to Ajax and three tonight. That tells the story. Maybe it's just one of these years when we need to take stock in terms of assessing how we should approach games at home."
The Reds face a real uphill battle to stay in the Europa League as next week's visit to San Mames is daunting one and a two-goal victory will be required unless Sir Alex's side can at least match Athletic's three away goals.
"I’m going to need to assess our games because we’re maybe finding it difficult to marry the two competitions together, playing Thursdays and Sundays as we do normally with the Champions League," added the manager. "I think we can win the game okay.
"I made the point during the week that we’re now starting to get players back. To marry the two together successfully, I think you need the full