Ajax had been fancied to test United but they unable to impose themselves on the game, and Bosz has put that down to Mourinho's 'no-risk' strategy.
The 53-year-old told reporters: "I'm disappointed because you play finals to win them. We didn't. I haven't seen the Ajax that I'm used to, which means good football, high pressing and being dominant.
"High pressing was difficult because Man United only played long balls and didn't take any risks in the build-up. I think it was a boring game. There were no chances from both sides."
The win for United earned the club a place in next season's Champions League.