The Red Devils agreed an initial £75 million fee with Everton for the Belgium international in the summer, and he's hit the ground running at Old Trafford, netting 10 goals in nine games already in 2017-18.
Speaking on ITV's UEFA Champions League Highlights show (h/t Aaron Flanagan of the Daily Mirror), Keane praised the team's new No. 9.
"He is a good player at a good age," he said after United's 4-1 win in Russia. "He knows the Premier League. He was a bargain at £75 million. It was obvious he was going to score goals. He has got off to a good start. He comes across well and seems like a decent character."
United moved quickly to get Lukaku on board this summer, and from that point on, prices escalated, triggered by Neymar's world-record, £200 million move to Paris Saint-Germain. Football writer Ell Bretland suggested the Toffees should have got more for the 24-year-old:
Ell Bretland @EllBretland
Everton didn't get near enough money for Romelu Lukaku. #EFC #MUFC2017-9-27 19:45:08
The striker is one of the most natural finishers in the game. At Everton last season, when chances fell to him, he was lethal in the penalty area; and with the likes of Henrikh Mkhitaryan and Paul Pogba providing support, opportunities are coming more frequently.
Goals are flowing as a result, with his two in Moscow key to United sewing up an important victory early on.
As Samuel Luckhurst of the Manchester Evening News noted, Lukaku's start at United is incredible when compared to another goalscoring legend:
Samuel Luckhurst @samuelluckhurst
To put it into context, Van Nistelrooy didn't score his 10th #mufc goal until November. And he went on to get 36 in 2001-02. #Lukaku2017-9-27 19:42:52
The forward's attitude has been exemplary too. "I just want to win trophies," he told BT Sport (h/t Rob Dawson of the ESPN.co.uk) when asked whether he had a goal target in mind for the season.
Sometimes, footballers can get lost when they make the step up to a club such as United and fail to show their best under the spotlight at Old Trafford. Lukaku appears to have made this move at the perfect time in his career.
The club's supporters will be desperate to see him continue his goalscoring glut, particularly against rivals Liverpool on October 14.