BILLY STARK admits Tony Watt must get fitter if he is to fulfil his potential.

The Celtic striker was criticised by Lierse boss Stanley Menzo after his debut for the Belgian club following his loan switch from Parkhead.

And Stark has now left him out of his Scotland Under-21 squad for the matches with Slovakia and Georgia.

Stark said: "It has been highlighted over the last week by the Belgium manager. He does need to work on his fitness.

"Every time he has come to us he has flagged up that his fitness is not what it should be and it is very difficult for us to gain that fitness.

"I just felt that in the last couple of games, maybe that has shown up a wee bit so we go with ones that are in a good, fit state.

"I will speak to him. I think it will bother him.

"One thing is, he has always shown up and shown a keenness to be involved."

While Watt has missed out on a cap, the national boss has handed former Queen's Park star Andrew Robertson his first call-up for the crucial Euro Champions qualifiers.

The left-back has thrived in the SPFL Premiership with Dundee United this season.

Stark said: "I saw Andy with Queen's Park a few times last season and I was really impressed with him.

"Obviously you are making a judgment of a player playing in the Third Division against some who are playing higher up.

"But since he has gone to Dundee United he has hit the ground running and he has grasped the left-back position from the off.

"In the first televised game against Partick Thistle he was impressive.

"We have been keeping an eye on him since then and he has been rewarded by getting in and it has been well-deserved."