Scotland head coach Andy Robinson this week named a side showing 14 changes from the team which beat Ireland 10-6 a fortnight ago, with Graeme Morrison the only member of the starting line-up to remain for the final World Cup warm-up match.
Barclay will start at openside in his first competitive action since the RBS 6 Nations win over Italy at Murrayfield on March 19.
And he is eager to show his worth after being withdrawn from action for Glasgow Warriors to preserve him for New Zealand.
The 24-year-old told Press Association Sport: "I'm really desperate to play well tomorrow, to go out and really enjoy it, get my hands on the ball, make some good tackles.
"It's exciting just to get to play rugby again. I've not played for five months, I'm a bit nervous, but looking forward to it."
Barclay feels the benefit of the rest.
He added: "I came back niggle free, I got through pre-season pretty well.
"I feel like I've got stronger, fitter, faster - all the goals you're looking for. I've got to see where I am tomorrow."
Prior to the Ireland Test on August 6, Barclay had featured in 15 straight matches for Scotland and in 17 of Robinson's 18 fixtures in charge since autumn 2009.
While Barclay appears the number one choice at openside flanker, he is taking nothing for granted.
Back row is one of the most competitive areas of the Scotland squad and it will be intriguing to see who Robinson decides to leave behind when he announces his 30-man World Cup squad on Monday.
Barclay sees tomorrow as a chance to prove his standing as first choice number seven, with Ross Rennie having made his case in his first Scotland start against Ireland.
Barclay added: "There seems to be a lot of quality back rows around and throughout the squad there's genuine competition.
"All I can do is play well tomorrow and hopefully get on the plane.
"It's up to the coaches - all I can do is try and prove I'm the best in my position."
Rennie's presence in the squad is keeping Barclay on his toes.
He added: "He's a quality player and it's good to have someone to put pressure on. We push each other forward.
"Whenever I've watched Edinburgh play he's always been one of their best players.
"I'm always very aware of him, even if he hasn't had that many chances recently because of injuries."
Italy's pack and back row, in particular, will provide a stern test tomorrow, but Barclay believes, along with Kelly Brown and Richie Vernon and the rest of the Scotland team, that the hosts are up to the challenge.
He added: "They've got a pretty good back row. (Sergio) Parisse is world class - we'll have to try and shut him down.
"But we've got a pretty good unit and I think our pack's been functioning well all week."
Robinson has decisions to make this weekend over the final places in his squad.
But his primary focus is the Test match and victory over Italy.
The head coach said: "I love playing against the Italians - they test you in a number of different ways.
"Physically up front it's a great challenge to have, but also the Italians are underestimated in their back play.
"I think they have some real quality in their backs - (Andrea) Masi is a fantastic rugby player, (Gonzalo) Canale in the centre has real pace and guile and (Mirco) Bergamasco scores a lot of tries.
"One of the key elements for us is to keep challenging the line and creating the line breaks and to finish those off.
"We've also got to respect they have a very good scramble defence now and it is important we don't take anything for granted and we get the support through."
Scotland: R Lamont (Toulon); M Evans (Castres), N De Luca (Edinburgh), G Morrison (Glasgow Warriors), S Danielli (Ulster); D Parks (Cardiff Blues), M Blair (Edinburgh); A Dickinson (Sale Sharks), S Lawson (Gloucester), M Low (Glasgow Warriors), N Hines (Clermont Ferrand), A Kellock (capt, Glasgow Warriors), K Brown (Saracens), R Vernon (Sale Sharks), J Barclay (Glasgow Warriors).
Replacements: D Hall (Glasgow Warriors), E Murray (Newcastle), R Gray (Glasgow Warriors), R Rennie (Edinburgh), C Cusiter (Glasgow Warriors), R Jackson (Glasgow Warriors), N Walker (Ospreys).
Italy: A Masi; T Benvenuti (Benetton Treviso), G Canale (Clermont-Auvergne), Gonzalo Garcia (Benetton Treviso), Mirco Bergamasco (Racing-Metro Paris); L Orquera, F Semenzato (Benetton Treviso); A Lo Cicero (Racing-Metro Paris), L Ghiraldini (Benetton Treviso), M Castrogiovanni (Leicester), CA Del Fava, C Van Zyl (Benetton Treviso), P Derbyshire (Benetton Treviso), R Barbieri (Benetton Treviso), S Parisse (Stade Francais, capt).
Replacements: T D'Apice, L Cittadini (Benetton Treviso), M Bortolami, A Zanni (Benetton Treviso), P Canavosio (Cammi Calvisano), R Bocchino (Estra I Cavalieri Prato), M Pratichetti.
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