THINGS haven't exactly run according to plan for James Forrest this season, but at least the 21-year-old has been able to take out his frustration in the weights room.

The Celtic winger has bulked up his upper body in the gym while recuperating from nagging hamstring problems in the last three months, and hopes his new physique can help him prove the archetypal "new signing" for the second half of the season who doesn't cost a penny.

"James has trained very hard and his rehab went fantastically well," said manager Neil Lennon. "You can see the development in his body shape. He has a really strong upper body and just looks solid.

"He's also always been very strong in the hip and pelvic area – he's just been prone to pick up these niggly injuries which sort of curtail him.

"It has been frustrating for him, but he hasn't let that frustration get the better of him. I think he'll be better, stronger. How old is he now? 21? He will just be itching to play and I think people better be wary of him."

Forrest is primed to make an impact for both club and country. Wingers already look like playing a key role in the new Scotland regime, and when Gordon Strachan turned up at Celtic Park in midweek primarily to check on the player's progress, he was able to quiz him in person.

Forrest, too young to have worked with Strachan at Celtic, was seated nearby in the stands. A brief chat between player and manager got the new Scotland boss the answers he needed. Not only has Forrest trained all week with no problems after a recurrence of hamstring trouble against Ross County at the end of last year, a bit of game-time for Scotland against Estonia on Wednesday might help him get up to speed for the challenges of Inverness and Juventus.

"I saw him at the Celtic game the other night, he maybe thought I was going to be playing, but I was up in the stand instead," Forrest said. "He didn't say much, he was just watching the game. I'm delighted I'm in the squad but a bit shocked as well.

"I came back in December for a week but in the first game I played I did my hamstring again lower down. Juventus is a big game but because I've been out for so long I just want to be back playing in any game."

Having seen him tearing up youth team defences when he first encountered him with his development squad, Lennon will always take a special pride in Forrest's progress. He is important enough to the club already to dictate what formation Celtic play both domestically and in the Champions League. As shown by his match-winning turn from the bench in Moscow, he can be a crucial part of the manager's tool-kit for the forthcoming matches against Juventus.

"I know things have been going well for us, including the qualification in the Champions League," Lennon said. "But eventually you do need players like James. We've had to sort of change the shape of the team, play a diamond, because we've had no natural width.

"We'll go with the three sometimes too because we've got no natural width. But our favoured formation is either 4-4-2 or 4-2-3-1 and with Jamesy back in we'll use it a lot more now."

What sets Forrest apart from other wide players is his knack for and willingness to do the muckier parts of the winger's role, even if the unfortunate image many will recall this season is of him losing Jordi Alba in his own six yard-box in the last minute at the Camp Nou.

"He is very good defensively," said Lennon. "We played Lyon in pre-season down at the Emirates just after I'd got the job and he was playing in front of Cha [Du-Ri]. There were times when he was directing Cha defensively!

"He's also a very fit boy. You ask a lot of them – I mean to run 40-50 yards with the ball consistently is a big physical challenge. If you do that four or five times in a game it can be demanding."

Even more priceless is Forrest's ability to respond positively to criticism. "He's one of those kids I can go through and I get the response," said Lennon. "He doesn't go under with criticism, he actually goes the other way. We were playing in the League Cup at Hibs last year and I had a right go at him at half-time: I gave it to him.

"Then he went out in the second half and was brilliant, scoring two of our four. You could also think of Kilmarnock last season. Whenever we've needed him in big games he's produced. He's a huge asset for us because of his quality and his pace. At European level you need pace and he's got that in abundance."

Raith Rovers in the Scottish Cup today will be too soon for Forrest to start, but he won't have to bide his time for too much longer. "We'll just try to break him in," Lennon said. "Maybe he'll get some game-time on Sunday, then Scotland, then we'll consider him for a start in Inverness."