The Portuguese coach, who replaced the Dutchman at United in the summer of 2016, has often been accused of setting his teams up defensively in big matches.
On Sunday, the Red Devils struggled to muster an attacking threat as they were outplayed by their neighbours Manchester City in a 2-1 defeat at Old Trafford.
During Van Gaal's two years at the North-West outfit, in which he won the FA Cup, the 66-year-old's methods were criticised, but he believes that he did a better job than Mourinho.
"I would rather watch City play than United," The Mirror quotes the former Bayern Munich boss as saying. "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 all right. 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 are 11 points behind leaders City in the Premier League title race.