Ruud van Nistelrooy said he was sad to see Erik ten Hag fired by Manchester United on Monday, but the club's former striker is honoured to step in as interim manager for as long as required.
Dutch coach Ten Hag was sacked on Monday with the team languishing in 14th in the Premier League standings.
The 48-year-old Van Nistelrooy joined Ten Hag's staff as an assistant in July. He was appointed interim manager on Monday, taking charge for Wednesday's League Cup match against Leicester City at Old Trafford.
"Erik ten Hag brought me back to Manchester United in the summer and, while I have only been part of the coaching staff here for a few months, I will always be grateful to him for giving me the opportunity, and I am saddened to see him leave," the former Netherlands international said on the team's website.
"When I returned in the summer as Erik's assistant, it was because I believe that Manchester United can climb back to the levels that I knew here as a player. I still have that belief, but it will take time and a lot of hard work.
"Even on an interim basis, it is a great honour to manage the club I love for however long I am asked to do so. I can promise that I will continue to give my all, in whatever capacity, to try to turn around our fortunes."
While Van Nistelrooy was considered a contender for the permanent position, Sporting boss Ruben Amorim has emerged as the front-runner, with United telling the Lisbon-based club on Tuesday that they are prepared to pay the €10-million release clause.