Mackay told BBC Three Counties Radio: "I'd rather have a centre forward who's scoring goals rather than not, good players are going to attract attention.
"Danny's here and he's enjoying his football, he's one of my leaders in my team at the moment."
The frontman, who signed a new two-year deal in September, has already matched his goal total from last season, when he found the net 14 times in his debut campaign at Vicarage Road after arriving from Carlisle.
Mackay is expecting reports linking Graham away from Watford to emerge, but insists the former Middlesbrough trainee is focused on scoring more goals for the Hornets.
"He's someone that has a target in terms of goals at the end of the season," he said.
"He has kept on from last season, coming into the Championship, and is now looking like a very difficult centre forward to play against."
The striker is still two goals shy of his best ever haul for a single campaign, scoring 16 times for Carlisle in the 2007-08 and 2008-09 campaigns.
Graham said: "I just want to get as many as I can, I've matched last year but in terms of league goals I'm still two off what I had last year.
"There's still goals to be had and I'm going to work hard in training on the finishing side and hopefully I can stick a few more away before the end of the season."