The versatile back, previously of Dumfries, is now in his third season at Millbrae and will pair up with Peter Jericevich in the half-back positions.
With both teams having won their opening-day fixtures last week and the dramatic Scottish Cup final between the two in April – which Ayr won in extra-time – still fresh in everyone’s minds, it promises to be exciting.
To accommodate Diez, Ross Curle moves to centre. On the Ayr bench, exciting young back-row Callum Templeton continues his return from long-term injury on his 20th birthday.
Ayr head coach Peter Laverie said: “Following a pleasing bonus point win last Saturday at Stirling, you can’t ask for a bigger challenge than having Melrose next. We identified key areas to work on this week and the players have put in a good shift this week to prepare. John Dalziel is an excellent coach and he will have his players ready to provide a physical challenge and an intense contest.”
Dalziel added: “This is a new season and both teams will be looking forward not back. It’s set up to be another cracker.”
For the visitors, Edinburgh Rugby pro Perry Parker starts in the second-row after impressing off the bench last week.
It is also Ladies Day at Millbrae today, with more than 200 people attending, so they will add a bit of glamour to what was already set to be a big crowd.
Early season leaders Gala head to Edinburgh to take on Heriot’s, with prop Luke Pettie going to be out for a long spell on the sidelines after suffering a broken leg.
Gala head coach George Graham said: “Every other time we have played Heriot’s they have been tight affairs, with both packs showing up well. I expect nothing other than a very tough encounter.”
Heriot’s were disappointed with their first-half showing at the Greenyards last week and will be keen to improve.
In a strong-looking backline, Max Nimmo gets the nod over Harry Boisseau on the wing, while Glasgow Warriors pro Kevin Bryce, who played at the club for the last few seasons, is released for a bench spot.
Aberdeen Grammar and Edinburgh Accies both scored victories last week and meet at Rubislaw.
A number of players will make their home debuts for Grammar, while Calum Gordon comes in on the wing, with Erlend Oag missing out with a hamstring strain.
Accies head north with an unchanged starting XV, head coach Jonny Else happy with the way his side battled to see off Glasgow Hawks on the opening day.
Hawks will be smarting after that one point loss and today host Hawick. The two sides had great tussles in the RBS National League last term, drawing 16-16 at Old Anniesland and a late Lee Armstrong try seeing Hawick win 11-6 at Mansfield Park. Expect another nip-and-tuck affair this afternoon.
Currie host Stirling County, with both clubs looking for their first wins of 2013-14. The last time County visited Malleny Park in February, a hat-trick of tries in the last seven minutes by Rory Hughes helped the visitors come back to win 39-21.