Jose Mourinho's team need four points from their final three games to secure their place as runners-up to Manchester City while victory over Chelsea in next month's FA Cup final would seal a second successive season with a trophy.
Although the United boss is not yet sure of the extent of Lukaku's injury, he said the Belgium global may be unable to make May 19's final against former club Chelsea at Wembley.
"I think that we always say that the most important one is the one that we win", he said. "Or if he's not OK". He wants to play all the time. He is never exhausted, so when he has small things he doesn't care, he wants to go. He still wants to go.
"To say immediately "change" is because he couldn't play".
"We have to continue, we have three games to prepare for the final".
The win moved United five points clear of third-placed Liverpool and confirmed their place in next season's Champions League, whereas Arsenal remain without an away Premier League point in 2018. "We know that. I think he will do well at Arsenal".
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"I am almost 33 now and getting on a bit but I still have that hunger and desire to go out and play in every game. I wait a bit more", Mourinho said, when asked about the situation.
And Mata was concerned about the 24-year-old being forced off, especially because of the impact he has had this season.
The United midfielder wrote on his blog juanmata8.com: "The bad news was Lukaku's injury".
"I want him to stay, the club wants him to stay, I think he will stay, it is almost there", he said. "He is (and he's been) a very important player for us this season, I hope it's nothing serious and he can recover soon", Mata wrote in his weekly blog.
"We need four points now and there are three games left in the league".