Lupul fell on Howard while scoring at 6:11 of the third, and the Wings' goaltender was forced to leave. Hudler broke the tie 38 seconds later by beating James Reimer with a slap shot from the top of the left circle that caught the far post and went in. Reserve goaltender Joey MacDonald stopped all seven shots he faced for the win, which snapped Detroit's three-game losing streak.
"It doesn't appear to be anything significant," Wings GM Ken Holland said about Howard's injury. "He's day-to-day, probably sprain or a strain."
He added that Howard would have a precautionary MRI and would only say it was an "upper-body" injury.
Backup goalie Chris Osgood is on the long-term injured list because he's been slow to heal after sports hernia surgery in January. He can't be activated until only two games remain in the regular season.
Until Howard is ready to go, MacDonald is the Wings' No. 1 goaltender.
"It's one of those things. You got to be ready and I thought our guys did a great job," said MacDonald. "We didn't give them a whole lot in the third. I had one good save off a faceoff that kind of got me into the game."
The loss was devastating to the Leafs' playoff hopes. They now trail eighth-place Buffalo by seven points with six games remaining. The Sabres come to Air Canada Centre on Tuesday in a game the Leafs have to have.
"Big loss for our team in the standings but we have to respond," Leafs captain Dion Phaneuf said.
Hudler's blast past Reimer's glove was his 10th goal of the season.
"Turnover ... Z (Henrik Zetterberg) had it on his stick, saw me right away," Hudler said. "I wasn't thinking because I knew right away what I'm gonna do, and thank God it went in."
Todd Bertuzzi, back after missing a game with a sore back, added an empty-net goal in the final minute.
Lupul tied it when he swept the puck in while lying on Howard for his 11th of the season.
"The puck actually hit me in the stomach. I was able to see it as I was going to the ice and put it in," Lupul said. "I had my eyes on the puck ... I didn't think I was in the crease. Phil (Kessel) was shooting, so he (Howard) was coming out."
Holland thought the goal should have been waved off.
"I thought it should have been no goal, no foul, no penalty," Holland said.
Tomas Holmstrom gave Detroit a 2-1 lead 9:17 into the second period with a power-play goal, tipping Kronwall's shot from the point behind Reimer for his 15th of the season.
Zetterberg opened the scoring 5:05 on a power play into the game when he beat Reimer with a backhander from the bottom of the left circle for his 22nd of the season. Nazem Kadri, the Leafs' top pick in the 2009 Entry Draft, tied it with a wrister from the right circle past a screened Howard at 17:56 for his second NHL goal.
Material from team media and wire services was used in this report.