The Dutchman, who was in charge at Old Trafford between 2014 and 2016 before being succeeded by Mourinho, also said he'd rather watch Manchester City than United following the Sky Blues' 2-1 victory at Old Trafford in the Manchester derby on Sunday, per David McDonnell in the Mirror:
"I would rather watch City play than United. You need quality in a squad and it's clear City have a better squad. If you ask me how did I do at United, I will say it was my best year ever, given the circumstances I was working under.
"We played football that was quite alright. But it's not football that is appreciated in England. And yet, right now, looking at United, I have to conclude Mourinho is not being criticised while it's far more boring football. What United produce now is defensive football. I always played attacking football. The proof is that the opposition were always parking the bus. They don’t do that now because Jose Mourinho plays so defensive."
United's defeat to City saw them drop 11 points back from their local rivals at the top of the Premier League table.
They are still second, though, three points clear of defending champions Chelsea in third and having netted 36 goals in 16 league games this term, just 13 fewer than in the entire 2015-16 campaign, Van Gaal's second in charge of the Red Devils.
Van Gaal succeeded David Moyes at United and led the Manchester giants to a fourth-place finish in the Premier League in 2014-15 and fifth place a year later.
He won the 2015-16 FA Cup while in charge of the Red Devils but was then promptly sacked and replaced by Mourinho.
The former Ajax, Barcelona and Bayern Munich boss was widely criticised during his two-year reign at Old Trafford for instilling an unexciting brand of football.
United legend Paul Scholes regularly criticised Van Gaal's negative tactics during the Dutchman's tenure and said his team was not one he would have enjoyed playing in, per BBC Radio Manchester (via BBC Sport).
Mourinho has faced similar criticism during his time at United, with the Portuguese being accused, in particular, of playing negatively when away at top-six rivals, per MailOnline's Alex Martin.
At least this season, though, United have at times looked to be returning to the swashbuckling attacking style that defined them in the Sir Alex Ferguson era.
United have scored four goals in eight separate matches in 2017-18, something they managed just four times in Van Gaal's entire tenure.
There will be few who dispute the Dutch manager's claim that City are a better side than United at the moment, but then Pep Guardiola's side are arguably currently the best team in Europe.
Van Gaal's claim that Mourinho's United are more boring than his Red Devils sides is likely to be met with scepticism, though.