He's back on the shelf now with an upper-body injury and it's unclear when he'll return to the lineup.
Lupul was not at practice Friday and did not make the trip to New Jersey for Saturday's game against the Devils. The 29-year-old forward appeared to take a blow to the head in Thursday's loss to Philadelphia, which ended Toronto's three-game winning streak.
Head coach Randy Carlyle was asked if Lupul suffered a concussion, but said it was too early to provide specifics on injury details.
"Obviously everybody saw the hit, he got squeezed out, he got dazed, he didn't feel very good," Carlyle said. "So we just said we're going to send him for an examination this morning and he had an evaluation done last night and they just say it's day to day."
Lupul was sandwiched by Jay Rosehill and Adam Hall on the first-period play. He got back to his feet, but was wobbly and had to be helped to the bench.
He left for the locker-room a short time later and did not return.
Lupul was recently named NHL first star of the week and was on a blistering two-point-a-game pace since coming back from the arm injury.
"It's tough, (Lupul) is an important piece to this puzzle," linemate Nazem Kadri said. "Definitely for us, we've had some chemistry and that's going to be missed but we've got a couple guys back in the lineup.
"(Lupul) has missed a good portion of the season so far and we've found a way for other guys to step up and take initiative and really get the job done."
The issue of head injuries — along with the dreaded "concussion" word — came up as Carlyle took questions from reporters after the one-hour practice.
"No, that's a bad word," Carlyle said. "We don't use that word until we're 100 per cent sure on any of the situations medically. The term concussion in today's sporting world, you want to make sure you're 100 per cent sure before you start using that word."
Clarke MacArthur skated on Lupul's line with Kadri and Nikolai Kulemin during the morning practice at the MasterCard Centre.
"(Lupul) does such a great job of coming back and being in shape and pucks are going in for him and then you get banged up again," MacArthur said. "So hopefully it's minor."
Lupul has had injury problems throughout his career.
He missed the end of last season after suffering a shoulder injury and sat out several months with a back injury and subsequent blood infection a few years ago.
"It's terrible, it's really unfortunate, especially the way he's getting hurt," Kadri said. "It seems like harmless plays really and he just gets the wrong side of them.
"But he's a big contributor and I know he's going to come back even stronger."
Lupul's recent offensive production was a big reason why Toronto had recorded points in eight straight games before the 5-3 loss to the Flyers at Air Canada Centre.
"He's an impact player for us, there's no way to mask it," Carlyle said. "Right now we have to have other people sharing some of the responsibility that he's been shoring up for us. It's as simple as that."
MacArthur and forward Leo Komarov could be back in action Saturday after missing some recent games with undisclosed upper-body injuries.
Entering play Friday, Toronto was tied with the Ottawa Senators for fifth place in the Eastern Conference standings with 44 points. The Senators have a game in hand.
The Maple Leafs have an important stretch of games coming up as they strive to make their first playoff appearance since 2004. Toronto has a home-and-home series on tap with the New York Rangers next week followed by a home game against Montreal.
But first up is New Jersey, a team that's right on the post-season bubble.
"Always a dangerous team," Carlyle said of the Devils. "You know the type of game it's going to be — tight checking. They're in a battle, they're looking for playoff positioning.
"In reality, that's every team we play right now."
Lupul, a native of Fort Saskatchewan, Alta., signed a US$26.25-million, five-year contract extension last January. He recorded 25 goals and a career-high 42 assists and 67 points in 66 games last season.
Toronto acquired Lupul and defenceman Jake Gardiner from the Anaheim Ducks in exchange for Francois Beauchemin in February 2011. In 525 career NHL regular-season games, Lupul has 159 goals and 178 assists. He has 23 points (14-9) in 39 career playoff games.
Notes: Maple Leafs defenceman Ryan O'Byrne also practised Friday. He was acquired from the Colorado Avalanche on Wednesday but did not dress against the Flyers.