Ralf Rangnick questioned Manchester United's body language after Cristiano Ronaldo's penalty clinched a "lucky" 1-0 victory at struggling Norwich on Saturday.
Ronaldo won and converted the second half spot-kick that gave Rangnick a second successive Premier League victory since he replaced the sacked Ole Gunnar Solskjaer.
United found it difficult to impose themselves for long periods and it took several fine saves from David de Gea to keep them on level terms before Ronaldo's 13th goal since rejoining the club from Juventus in August.
"It was a very intense game, very physical," Rangnick said. "We need to be able to compete on that kind of level. There's still room for improvement in this area.
"The second half was a little bit better but at the end we were lucky to have a clean sheet. It's another clean sheet but still a lot of work to do.
"It's also a question of body language and physicality and this was one of the major issues that we could have done better."
Rangnick's side sit in fifth place in the Premier League, level on points with fourth-placed West Ham, as the German aims to haul his club back into the Champions League places.
While United must improve significantly on this mediocre display against the Premier League's bottom club, Rangnick will take heart from successive clean sheets -- a stark contrast to the woeful defending seen in the 4-1 loss at Watford that ended Solskjaer's reign.
Rangnick, who kept faith with the team that beat Crystal Palace 1-0 in his first match last weekend, wants to introduce a high-tempo pressing style to United.
But after making 11 changes from the midweek Champions League draw with Young Boys, there was a noticable lack of energy about United at times.
Norwich were without several players after boss Dean Smith hinted at coronavirus concerns in the squad on Friday, but they still matched United stride for stride.
United were inches away from an early lead when Alex Telles' free-kick deflected off the Norwich wall and cannoned off the crossbar.
While United had the star power and greater resources, there was nothing to separate two misfiring teams in the first half.
Teemu Pukki should have underlined that point when he let the ball escape his control after Lukas Rupp picked out the unmarked Norwich striker six yards from goal.
After a subdued start, Ronaldo wriggled onto Scott McTominay's pass for a fierce drive that Tim Krul pushed away.
Harry Maguire's diving header from Jadon Sancho's cross forced a back-pedalling tip over from Krul.
Pukki threatened soon after the restart with a swift turn and shot that had De Gea at full stretch to save.
Marcus Rashford hadn't scored since the end of October and the United forward was left frustrated again when his low drive rippled into the side-netting.
Rangnick responded to United's lethargic display by sending on Mason Greenwood for Sancho.
There was a scare for United when their defender Victor Lindelof was forced off after falling to the turf holding his chest.
Lindelof seemed to have recovered as he walked off and, as ever, it was Ronaldo who was the catalyst as United finally made the breakthrough in the 75th minute.
Challenging for McTominay's cross, Ronaldo was dragged down by Max Aarons and the Portugal striker picked himself up to drill the penalty past Krul.
Pierre Lees-Melou almost grabbed an immediate equaliser but the Norwich substitute's header was clawed away by the inspired De Gea.
Norwich could feel aggrieved that they didn't earn a point as Billy Gilmour's goalbound effort was deflected wide by Eric Bailly's last-ditch block.