Manchester United’s push to appoint Ruben Amorim as their manager is being held up by negotiations with Sporting regarding his 30-day notice period.

United have informed the Portuguese champions of an intent to pay Amorim’s €10m (£8.3m) release clause and want him in place for Sunday’s visit of Chelsea but this may not be possible unless there is a breakthrough in discussions.

Related: Amorim saved Sporting from despair but United marks a whole new test | David Hytner

United play six games across the next 30 days, then host Everton in the Premier League on 1 December. There is confidence within the United executive that the 39-year-old will be appointed well before then.

After Sporting beat Nacional in the Portuguese cup on Tuesday, Amorim said: “Nobody knows if it was my last match. It’s something still not decided … When and if I take that decision I will say it.” Sporting play at home to Estrela Amadora in the league on Friday night.

Before United took on Leicester in the Carabao Cup fourth round on Wednesday, United’s interim manager and former player Ruud van Nistelrooy spoke about Erik ten Hag, who was sacked by the club on Monday.

The former United and Netherlands striker said: “It’s mixed feelings – sad to see Erik go. He was the one who got me here. Our first conversation was about his passion for Man Utd. He did everything for this club.

“I’m asked to help the club – that’s the reason I’m here, to help, to build. That’s where I take pride in leading the team today. We are to stabilise and go out for training and get to work. The team talk was about the opponent and ourselves, what it is about to play at Old Trafford in front of these wonderful fans.”

Van Nistelrooy hoped to inspire the squad to “lots of possession and attacks – that’s what Man Utd is about. I’ve tried to share that with the players.

“The most important thing is passion and energy and to get the crowd going and give them a great night of football. It’s a big day.”