It's a tricky one, I've found... the old adage of "form is temporary, class is permanent" doesn't necessarily seem to apply all the time in Hattrick.
As an example, I've got my two 3.5* defenders. One - solid defending, passable experience (aged 31), and his form has only ever been solid or excellent, ever since I bought him a year ago as a 30yr old for £98K.
The other - formidable defending, 24 years old (poor experience, I think), and I bought him with wretched form for £365K at the beginning of the season. His form did get up to passable for 1 week, but has dipped back to inadequate.
However, I guess with Hattrick, it's all about potential. So, even though the older guy has been picked every week, because he churns out 3.5* performances every week - whilst the younger guy has only hit 3.5* a couple of times and so keeps getting dropped - I still don't regret spending 3.5 times as much for the younger (and unreliable) guy.
The latter is younger, and has a lot more potential... with his stamina up to solid or so (I'm embarking on a 7 week stamina training regime), and once he's been with me a bit longer, I'm hoping his form will rise in time.
So, the question is - can you afford your player now... and crucially, can you afford for him to be playing either in your reserves, or very poorly in the first team, whilst his form improves?
If the answer to those is "yes", and if you can't find another solid winger you'd rather have, then I'd say that there's a pretty reasonable argument for snapping him up...
Hope that helps,
Matt