Mourinho lamented United's laissez-faire approach in the second half, reminding his team that no lead is safe in Europe's premier competition.
"Until 2-0, we were very stable, we played with confidence, with patience, with good choices. We played quite well. After 2-0, everything changed," the 54-year-old told BT Sport. "We stopped playing seriously, we stopped making the right decisions on the pitch. We could have put ourselves in trouble.
"Fantasy football, PlayStation football, tricks ... I don't like it. You gamble a little bit. Probably the players thought the game was under control at 2-0, but football is football. You have to respect the opponent."
Basel troubled United in the past, eliminating the Red Devils from the Champions League group stage in 2011, but couldn't make them pay this time around.
Mourinho did shower individual praise on Fellaini, who's become a super sub of sorts for United.
"I keep saying the same, the same, the same. He's a player with special qualities. He gives me multiple areas of action. He is one of my most important players," Mourinho added.