Jones conceded a calamitous own goal during a recent defeat to Tottenham Hotspur, while Smalling was penalised for diving against Newcastle United on Sunday, with the Magpies going on to score the winning goal from the resulting free kick.
"They have got the best defensive record because they have the best goalkeeper in the world in David de Gea and Matic out in front who protects them really well," Neville told BBC Sport.
"But the last two away games, Smalling and Jones have been a disaster. Against Dele Alli and Harry Kane, they got bullied. Against Newcastle did they make many mistakes, no, but they created a nervousness and a tension.
"They were making decisions that were strange. They were making decisions all day where you were thinking: 'What are you doing?'"
United's defeat leaves them 16 points behind leaders Manchester City and now just two above third-placed Liverpool.