David Johnson retired from football in 2007 after an impressive career as a striker, with Brennan just six-years old at that time.
The forward is wanted by the likes of Newcastle, Tottenham, Everton and Brentford.
So even if Forest don't win promotion in the play-off final against Huddersfield at Wembley, he will most likely be in the top-flight next year after a stellar season.
The Wales international was born in Nottingham and grew up idolising David Beckham.
He qualifies to play for Wales through his mother's family, who are from Rhayader.
Johnson was only five months old when Beckham scored that famous free-kick against Greece, but he wants to emulate his former hero with the red of Walez and help his country get to the World Cup.
Beckham actually knows the Forest star's dad as they were both at Old Trafford at the same time.
And the former Man United and Real Madrid superstar kept in constant contact with David and monitored Brennan's progress from afar.
The 21-year-old has been with Forest since he was eight, and he has progressed through the academy at the City Ground.
Eventually, after impressing for Forest's youth sides, Sabri Lamouchi handed him his debut in the Championship, Carabao Cup and the FA Cup.
Unfortunately, things didn't work out for Lamouchi and he was sacked.
In came Chris Hughton, and Johnson was loaned out.
Now, this could have been a blow for a player who potentially thought he would be in the Forest first team last year.
But this loan move to Lincoln City under Michael Appleton proved to be the catalyst in his career.
He made 43 appearances for the Imps, in which he scored 11 goals and provided 13 assists, and Appleton was very impressed by him.
He said: "I'd be amazed if there's not more interest from the Premier League in the summer - by three or four clubs, if not more.
"There were a few things which stood out with Brennan. He's got pace and he's very direct. Whenever he received the ball, even on his back foot, he'd look to turn and always go forward. So many players these days opt to play it square or go backwards but he always looked to be positive.
"The biggest compliment I can pay him is that I used him in five different positions. We played him either side of a striker, we played him as a number 9, as a 10 and even seven games where we played him as a No 8 in midfield.
"We knew we had someone who is not only very talented and grounded but also someone who is very adaptable, intelligent and has the right attitude. He was always among the two or three you'd struggle to get off the training pitch."
During that loan with Lincoln, Wales decided to act quickly and handed the forward his first cap.
The Dragons didn't want England to swoop or for Johnson to end up in a similar situation as his dad, as they tried to cap David in the 1990s only to find out he was ineligible.
Since then, Johnson has won nine caps for Wales and has impressed Joe Ledley: "Brennan's a very exciting young player who is most definitely ready now to make the step up.
"He's been fantastic for Nottingham Forest and you've seen this season what he is capable of doing.
"I think Robert Page had blended him into the squad at the right time. We'll see if he starts but if he's on the bench, I'm sure he can come onto the pitch and make an impact."
This could be a defining couple of weeks for Johnson, who can first help Forest into the Premier League, and then take Wales to the World Cup.
However, after returning from that successful loan spell, Forest endured an awful start to the year.
Luckily, Steve Cooper replaced Hughton and Johnson has been a revelation.
He has scored 18 goals and provided nine further assists as Forest have made it to the Championship play-off final on Sunday.
Even Roy Keane was blown away by the Welsh international when the Tricky Trees beat Arsenal in the FA Cup in January: "He's a talented kid.
"No doubt a lot of teams are looking at him and we'll see if Forest can hold on to him.
"I will be interested to see what he's like against a tricky team. He's got a bit of pace and he's tricky."
Ultimately, the sky appears to be the limit for a player bound for the Premier League this summer, with or without Forest.