The 18-year-old made his first team debut as he came on as a second half substitute for Pawel Brozek with the game at Fir Park goalless and scored two quick fire goals to help his side win 3-0.
The win keeps the champions 18 points clear of second-place Rangers ahead of next Sunday's Old Firm derby at Celtic Park and Mjallby admits Watt's impressive debut means he could be in the squad for the match.
"He is definitely going to be in our thoughts," said Mjallby.
"It was a sensational debut for Tony. If the boy comes on and scores twice for you in a very tough game against Motherwell then obviously he is going to be in our thoughts.
"We had Georgios Samaras out, Anthony Stokes didn't get on, we have Mo Bangura, Pawel Brozek and Gary Hooper, so competition is there. But we've had our eyes on him for some time. He has done really well for the under-19s and the development team.
"He's a very powerful boy and he likes to get into the box. That's really important because we're always going to be able to create chances so we need strikers who get in behind defences and get goals.
"The main thing now is to keep his feet on the ground. I'm quite sure he is going to have a great future. Sure there are still a few departments where he can improve but he's only 18.
"He's definitely in our plans for next season. The main thing for Tony is to improve all the time in training."
Watt, who signed from Airdrie last year, came on in the 59th minute and scored his first goal just over three minutes later when he volleyed in Victor Wanyama's cross.
Watt grabbed his second in the 66th minute when he sent a low angled drive into the bottom left hand corner to mark a memorable start to his Celtic career.
South Korean international Cha Du-Ri rounded off the win when he headed in substitute Paddy McCourt's cross in the 83rd minute.
Watt was delighted with his dream debut.
"I was just wanting to make my debut as it's what you dream of but to score twice makes it even more special, especially for a club like Celtic," the teenage striker said.
"There is no other feeling in the world like that, it's just remarkable. You couldn't wish for anything better."
Rangers, who were docked 10 points for entering administration in February, warmed up for next Sunday's Old Firm derby by dispatching Hearts 3-0 at Tynecastle on Saturday.
Sone Aluko gave Ally McCoist's side the lead in the 29th minute with a superbly struck free-kick before Andy Little added a second before the break.
Craig Beattie sent his penalty crashing off the crossbar after David Templeton had been tripped in the box before Little popped up with two minutes remaining to claim a double.
Source: AFP
Source: AFP