The short answer is no, you should not. Last season, Jered Weaver was the ace of the staff, despite one of the slowest average fastballs among righthanded pitchers, 88.7 MPH. That number doesn't seem all that slow, but the average four seam fastball thrown by righthanded pitchers in the majors last year was 92.1 (Lefthanders are about 2 MPH slower on average). Recent research published on the Hardball Times suggests that the slower throwing pitchers tend to do worse than theirprojections, by about half a run of ERA. Weaver falls into this category.
While this is good research and may help to more accurately project pitchers in general, I've looked a bit deeper into the case of Weaver and don't think he has anything to worry about.
First of all, I was surprised to see Weaver among the slower throwing pitchers. He has enough games where he's throwing 92-93 MPH that I don't think of his fastball as slow, though he does have games where he can't dial it up and pitches closer to 86.
The 88.7 is an average of his 4 seam fastball, which is the most common pitch in the majors. Weaver also throws a cutter, and his average speed on this pitch is 89.8. Most pitchers throw the cutter with less velocity than the four seamer. This suggests he could throw the 4-seamer faster more often, but chooses not to, as a big part of his game is changing speeds.
Weaver varies the speed on his fastball. His best fastball on the season registered at 94.6 MPH, a difference of 5.9 MPH compared to his average. Most pitchers have about a 3-4 MPH range from their average to their best. My theory is that Weaver tops out at 94, and could throw 90-91 on average if he wanted to, but he's more effective throwing some slow pitches, then some fast pitches, and keeping the hitter from ever knowing what's coming. There were 281 pitchers who threw more than 500 4-seamers, and Weaver was in the top 10 in the difference between his average and his best. You can see it in his games, where he'll throw an 89 MPH fastball and the hitter looks as overmatched as on a Kevin Jepsen 98 MPH pitch. It's because Weaver can't be timed.
I looked at Weaver's game logs by average fastball speed, to see how he did in the 5 games where he threw the hardest, and the 5 games where he threw the slowest. When I started this I expected that his slow games would be the ones where he was hit the hardest, such as when he struggled in July. I was surprised to find something completely different. In his 5 slowest games, Weaver went 4-1 with a 2.06 ERA. He struck out 29 and walked 10 in 35 innings. In the 5 games where he threw the hardest. he also went 4-1, with a 1.97 ERA, 31 strikeouts and 7 walks in 32 innings.
The conclusion here is that Jered Weaver is a pitcher in the truest sense of the word, not a mere thrower. He has earned his spot as the Angels opening day starter.