The forward came off the bench at the break with the score locked at 1-1 to inspire the visitors to a 3-1 win on Sunday.
His first Premier League goal since September’s brace against Villa put Chelsea ahead and he then won an added-time penalty for Jorginho to score.
It was the Italian’s second from the spot after he cancelled out Reece James’ own goal in the first half.
Lukaku has been blighted by an ankle injury and then Covid-19 since October and while Tuchel was delighted with his impact, he said it was “not fair” to force him back into action for a full half.
Tuchel said: “There is no doubt he is super-important. Romelu will be a key player.
“Last time he started for us was in October, not because of quality or that we have any doubts we are better without him. He is unlucky to get injured.
“When he was struggling to come back in the schedule we have, he caught Covid. It (45 minutes) was more than the fitness and medical department recommended for him but we thought we could take the risk and be more offensive.”
Yet Tuchel admitted his celebrations are tempered because of his concerns surrounding his players’ health, having also lost N’Golo Kante and Thiago Silva to injury.
He said: “It’s a big win but I’m always concerned for the health and safety of the players and that’s why you don’t see me fully excited.
“Normally you could catch huge momentum and push them but the only thing we are doing is checking on a daily basis who can be pushed.”
Speaking to Sky Sports before his press conference, Tuchel expressed his frustration with officials at the Premier League’s “green table”.
“I am very impressed with my players but super-concerned,” he said.
“Maybe we made a big mistake here letting players play after corona and one or two training sessions. But obviously they (the Premier League) made us play and so we play.
“This cannot be the right way. People at the green table and people in offices take these decisions. This is how it is.”
Villa impressed in the first half and went ahead after 28 minutes when James diverted Matt Targett’s left-wing cross over Edouard Mendy.
Six minutes later Chelsea levelled when Matty Cash dived in on Callum Hudson-Odoi in the area for Jorginho to score his first penalty.
Lukaku replaced defender Trevoh Chalobah at the break and instantly changed the game, with Chelsea more confident and dangerous.
He took just 11 minutes to score when he brushed off Tyrone Mings to glance in Hudson-Odoi’s cross.
Villa failed to deal with the Belgium international and lost their way after the break, with Lukaku running amok again in injury time.
He broke onto Hudson-Odoi’s ball and easily left Targett behind before Ezri Konsa brought him down and Jorginho added a third, and his second, from the spot.
Villa were without boss Steven Gerrard, who is isolating after testing positive for Covid, with assistant Gary McAllister leading the squad.
“I’ve been speaking to Steven ever since he was diagnosed. He is obviously very frustrated, it’s a game he wanted to be part of,” said McAllister, with Villa 10th.
“We are in this world of Zoom and there are ways of communicating. We were in constant contact with each other during the game.
“He is due to be back on Friday and we have four or five days to work and try to correct the things which didn’t go so well and look to build on what we’ve done.
“Even though I thought we played some good football, to get an end product I thought we lacked a bit of imagination. There has to be an outcome after your good play.”