That’s according to ex-Blues star Chris Sutton.
Matic was sold to United yesterday for £40m.
Chelsea have already secured a replacement in Tiemoue Bakayoko.
The Frenchman signed from Monaco in mid-July but Sutton still thinks his former club need more.
“He’s certainly a loss for Chelsea,” he told BBC Five Live when quizzed about the Matic deal.
“They’ll feel Bakayoko is a like-for-like replacement.
“I don’t think Chelsea have the strength in depth and they’ve got to compete in Europe this season.
“They are short certainly in that central midfield area.
“They’ve got Kante and (Tiemoue) Bakayoko but they’ll need more this season.”
Along with Matic, midfielders Ruben Loftus-Cheek and Nathaniel Chalobah have departed Chelsea this summer.
Loftus-Cheek has joined Crystal Palace on loan and Chalobah was bought by Watford for £5m.
