Matic, 29, left Stamford Bridge for Old Trafford for £40m this summer in a move that raised eyebrows within the football world. Many were left wondering why the Premier League champions were seemingly happy to strengthen a perceived rival for the title, and Le Saux believes the Serbian's departure has heavily contributed to the "real issue" Conte is having to contend with.
"I think the real issue they have is in the middle of the park and I can't understand why the club would let Nemanja Matic leave," Le Saux was quoted as saying by the Mirror.
"What Conte did so brilliantly last season was pick the perfect pairing for every game between [N'Golo] Kante, [Cesc] Fabregas and Matic and now with [Tiémoué] Bakayoko side-lined and Cesc suspended he's left with a huge hole to fill in midfield."
Chelsea are preparing to face London rivals Tottenham at Wembley on Sunday (20 August), and Le Saux, who is not the first former player publicly question Chelsea's decision to allow Matic join Manchester United, thinks Mauricio Pochettino's men are more than capable of exploiting Chelsea's midfield problems.
Tottenham's midfield boasts the attacking talents of Dele Alli and Christian Eriksen, two players who could wreak havoc against a troubled Chelsea side if Conte does not swiftly find a solution to his
"Conversely this is exactly where Tottenham can hurt the opposition the most; both Eriksen and Alli were sublime last week and Chelsea will have to come up with a quick answer for the dynamic midfield pairing if they're to come away with anything on Sunday."
Matic, who delivered a commanding performance on his Premier League debut for Manchester United, helped Chelsea defeat Tottenham when the two sides last met at Wembley in April.
The Serbia international scored a stunning goal to help the Blues reach the FA Cup at Spurs' expense, but will now be focusing on matters in Wales rather than Wembley as Manchester United prepare to travel to the Liberty Stadium to face Swansea City on Saturday lunchtime.