Hoolahan has joined West Brom on a short-term contract until January as a free agent following the departure of the 36-year-old attacking midfielder from Championship rivals Norwich at the end of last season.
The former Republic of Ireland international was on the books of the Canaries from 2008 until this summer and played with Holt at Carrrow Road.
The 37-year-old retired from professional football in August and is now working as a coach at Norwich.
Holt has taken to Twitter to send a message to new West Brom signing Hoolahan.
West Brom are playing in the Championship this season after getting relegated from the Premier League at the end of the 2017-18 campaign.
Darren Moore’s side are fifth in the league table at the moment with 11 points from seven matches, three points behind leaders Leeds United, who have played a fewer game.