Aston Villa have completed the signing of James Chester from West Brom for an undisclosed fee.
The defender has penned a four-year deal at Villa Park after just one season at The Hawthorns.
Villa are rebuilding following their relegation from the Premier League but have lost their opening two games this season.
Chester, meanwhile, has struggled for first-team opportunities at The Hawthorns since joining from Hull for £8million last summer, often played out of position and would again be behind Jonas Olsson, Jonny Evans and Gareth McAuley.
Di Matteo confirmed Villa were closing in on the 27-year-old and the Italian has now made him his fifth signing of the summer.
Chester impressed during Wales’ march to the Euro 2016 semi-finals in the summer and made the most challenges and interceptions of any players in the tournament with 36.
Villa host Rotherham in their first Sky Bet Championship home game of the season on Saturday having lost 1-0 to Sheffield Wednesday in the opener last weekend.
The Midlands club have already spent £12million on Ross McCormack in a bid to earn promotion at the first attempt – but the striker was withdrawn in the second half of Wednesday’s 3-1 EFL Cup defeat at Luton and is yet to score.
Crystal Palace midfielder Mile Jedinak is also on Di Matteo’s radar.
The Villa boss maintains he has seen signs of a positive change in approach from the squad, which cannot come quickly enough.
“Personally I have never experienced it, as a player or a manager. There is definitely a mindset which needs to be changed here, and we are doing better,” he said at a press conference, published on http://www.skysports.com.
“I can see the signs of improvement, but the best recipe is to get a positive result…the sooner the win comes, the better it is for the players’ confidence and the whole environment.”