2003 Chevy Malibu Engine Vibrations/hesitation at idle

I just received a tune-up in which I replaced the spark plugs, serpetine belt, fuel filter, upper and lower radiator hoses and pcv valve. The car ran like a top before this work was done. The engine now noticeably vibrates when in drive and not moving. It is fine in park and after I accelerate. The mechanic mentioned that the new belt may be too tight and I might need a tensioner and that should solve the problem. Is he correct...I really don't want to have to pay for a diagnostic test if this will fix the problem.