Ferguson helped inspire a turnaround from 2-0 down to win 4-2 at Kilmarnock in midweek.
But the former Rangers captain failed to repeat the trick after another shocking start saw the home side fall 2-0 down at Ibrox before half-time.
Rangers have now lost three consecutive home games for the first time since 2012.
"That level of performance, certainly in the first half, is nowhere near the standards that are needed," said Ferguson.
"I spoke to them in the dressing room, they understand it. Rangers fans can accept you not playing well. What Rangers fans can't accept is when your team gets run over the top of in the first 45 minutes."
Worse could be to come with Ferguson's men facing Jose Mourinho's Fenerbahce in the Europa League last 16 and a trip to Celtic in their next three games.
"It's clear they will suffer a heavy defeat," added Ferguson, describing what would happen without a major performance improvement.
"I'm not going to sit here and try and kid people on."
Luke Armstrong pounced on an error from Jack Butland to put Motherwell in front after nine minutes.
Tom Sparrow then drilled in a second as the visitors' new manager Michael Wimmer celebrated a famous win.
Cyriel Dessers pulled a goal back for the home side in the second half, but it was too little, too late on another embarrassing afternoon for the blue half of Glasgow.
Celtic are just three wins away from a fourth consecutive title after overcoming stiff resistence from St. Mirren.
Twice, the home team came from behind to level in Paisley.
A brilliant Declan John free-kick canceled out Jeffrey Schlupp's first Celtic goal.
Arne Engels' penalty put Brendan Rodgers' men back in front at half-time, only for Killian Phillips's strike to loop over Kasper Schmeichel early in the second half.
A triple substitution from Rodgers 25 minutes from time swung the momentum decisively in Celtic's favour.
Yang Hyun-Jun came off the bench to score twice with headers and in between set up Daizen Maeda for his 26th goal of the season.
"It's one of my favourite wins of the season," Rodgers.
"This was a massive step (towards the title) for us today. Third game in a week to come here and score five goals, that performance and result, gives us a 16 point lead.
"Our idea is to finish the season very strong."